I'd just like to add a few thoughts of mine.
Years ago, I was a mostly-vegetarian; I also did a very poor job of it - some of the respondents in this thread have described it well. I pretty much ate what wasn't meat. For me, that meant a lot of bread, honey, yogurt and chees, sweet things, not enough vegetables...not at all a well balanced diet.
I also worked with a vegan; she was overweight, often ate candy, and was fond of saying things like "I'm a vegan! I'm indestructible!" (as she tossed down some crystallized sugar based FD&C tinted, fake flavored garbage).
I'm not saying that my example and hers make up the bulk of your average vegetarian's or vegan's dietary practices and outlooks; but I've heard of it happening enough for it to be of concern.
I once worked with a vegan on a farm for a couple of months; he appeared very healthy - quite buff, in fact, a strong looking guy. He was an extreme minimalist in his diet; a great deal of peanut butter (for protein and fat I assume) on whole grain breads; veggies; fruits; and took all his foods plain (no seasoning).
Not long before he arrived at this farm, he was sent to the hospital emergency (so he said); he nearly died from something in his diet. Was it a deficiency? Lack of salt or something else? I'm not sure if/what he said. No matter, after he corrected it, he seemed active and capable and fine.
I've met two anorexics; they were both vegetarian, bordering on becoming vegan. Both seemed to have some sort of "spiritual" mindset. I've no idea if it was related.
Then again, there's the vegan bodybuilding website and forum, and Brendan Brazier, the vegan Ironman competitor, and probably plenty of healthy examples, too.
And there are, without doubt, plenty of unhealthy omnivores! Just take a look around your average North American city.
I guess what I'm trying to say is that you can mess up most ways of eating - and what works best for one person may not be the best for the next. Though I think there are probably some basic dietary rules which apply to all of us to varying degrees.
Me, personally? These days I consume a lot of animal protein (both beef and fish). How come? I've had food-related issues much of my life. I ate gluten for over twenty years without knowing it was damaging my guts until a lab test finally confirmed it. Same with most dairy, though fortunately I seem mostly okay with butter.
I would often get itchiness on my skin, foggy thinking, bad gas, or anything else when I ate foods. I guess you could call it food sensitivity, because it's happened with so many that it couldn't be an allergy to everything.
I think the aggravation to my intestine and immune system from the gluten, combined with an unwise consumption of too much sugar (mostly from dried fruit and honey and maple syrup, which, while "natural" are still highly concentrated), and the garbage I ate when I used to smoke pot, may have increased some issues and fostered a state of dysbiosis (I know some dispute that candida overgrowth can be a problem, but I've even had it come to the skin; I believe it can be a real issue for some). All these things aren't good for your digestion.
Recently, having been under a lot of stress, I started reacting to more and more foods. There was a point when everything made me itch and get a rash, even steamed broccoli. I fasted for five days and took glutamine and other nutrients to help heal the gut lining.
I'm still experiencing some issues but have improved.
Some might wonder why I'd eat so much animal protein if I had gut issues. Well, because grains and beans (which most people have been taught to think are very healthy foods) aggravate me more than most anything else it seems. Even gluten free grains. Beans have never been easy for me to digest at all times. And seeds, even soaked and dehydrated, started bloating the hell out of me. Eggs, I ate four of per morning for years on end, and may have developed a sensitivity to (they are one of the most allergenic foods); I'm taking a break from them, now. Most protein powders in quantity don't agree with me.
What does that leave me for protein? A few nuts - I seem moderately okay with almonds and pecans. Otherwise? Meat and fish.
And you know what? Since eating low heat dehydrated all-or-mostly grass fed beef, and wild caught salmon and sardines, along with a lot of fat, both saturated and not (butter, coconut oil for cooking, olive oil, and the fat in the beef), along with smaller amounts of fresh fruits and veggies, my symptoms have improved a lot.
I'm still having some issues, but not like before. The few carbs I eat, the less bloated and gassy I tend to feel. I know I've got work to do and I think I'm experiencing a lot of stuff coming to the fore that may have been years in the making, but it seems like this mostly "paleolithic", "primal", call it what you will, grain and legume and dairy (apart from butter), no concentrated sugar way of eating may be what my body needs. I hope it is.
There's a really interesting PDF out there; it's called "Cereal Grains: Humanity’s Double-Edged Sword". Definitely something to read, especially for those whose diet is grain heavy, or those who would adhere to the so-called food pyramid, which is very carb-heavy. I'm now of the mind that a grain-heavy diet is not what human evolution gave us the bodies for.
Here's a link to the PDF:
http://www.direct-ms.org/pdf/EvolutionPaleolithic/Cereal%20Sword.pdf
Do some people feel good when they eat grains? Oh, I'm sure they do, and more power to them! I sure enjoyed grains, and I still crave a nice slice of bread, slathered in butter; though now I just slather my steamed veggies in butter.
But what I didn't enjoy was being bloated and fuzzy and possibly damaging my digestion and having sugar crashes from eating so many carbs. There might be a lot of people eating tons of grains who experience the same things. It can be hard to go from running on a lot of carbs to utilizing dietary and stored fat for energy, but I've found it can feel like a more "even" sort of energy, once you're used to it.
The same I could say for beans.
And there are probably people whose systems can handle these foods better than others. I'm not sure if I'm one of them right now.
On a moral/ethical level, I'm not crazy about the idea of an animal feeling pain or dying so I can eat it. I don't really enjoy that - I try to be grateful for the meat I eat, and don't take well to the attitude that some meat eaters have (ie, "Rawr! I'm going to rip into my bloody piece of dead cow now! Yum!").
I find that attitude rude and disrespectful, and it earns no points with me; I wonder if it's a retaliatory response to more militant vegans using similar in-your-face tactics to someone eating meat...?
I'm not choosing to eat meat because it's meat; I'm choosing to eat it because I need foods that my body can digest, which don't cause me to break out in a rash, and because it seems what my body often craves. I'd love it if I could thrive as a raw vegan, eating tons of nuts and seeds and whatever else all day, but I feel that wouldn't be the best thing for me right now, though props to those for whom it works. I will not eat factory farmed meat.
On the note of vegetarians/vegans, I've heard enough stories of those who tried the whole vegan thing and became too thin and too unhealthy to continue - even by doing it "the right way". Maybe it just wasn't the best for them, personally, while for others it is.
I am a bit concerned about how much fish I eat; yes, it's lower mercury (wild salmon and sardines), but canned, and I've had it for a couple or so years, numerous times most days. Soon as I have a solid source of grass fed beef I'm reducing my fish intake big time and maybe doing a gentle heavy metal detox.
If I keep improving, I may well try to reintroduce certain foods. But for now, it's meat, fruits, veggies, fats, and a few nuts.
On the note of fat and cholesterol. I see it mentioned that vegan diets have little to no cholesterol and saturated fat. I do not believe this is a good thing! Cholesterol is not the demon it's been made out to be; nor is saturated fat. Our bodies also manufacture cholesterol - it's a vital component for our functioning. And saturated fat alone does not make you fat! I'm a fine example of this, being lean and reasonably muscular despite a high saturated fat intake; and any blood work that's ever been done on me has shown no lipid, liver, kidney, inflammation, or other such issue.
And I believe that it's inflammation, not cholesterol, which is the real concern for heart disease.
I think the bigger problems are an excessive intake of carbohydrates, especially from grains, gluten, rancid and industrially processed oils, high quantities of sugar, too much animal fat from cows eating too much grain (poor omega 6:3 ratio) and combining bad carbs with bad fats, along with poor intake of produce.
Give a person a diet high in clean saturated fats, good proteins, low in grains and lots of veggies and fruits, and they'll likely shed unwanted pounds. Feed 'em lots of grains and canola oil, limit protein, and most will probably gain fat and lose muscle mass. They may lose some scale-weight from pooping things out from all that fiber...some exceptions may apply, of course!
Anyone concerned with cholesterol, and considering taking, or taking statins, for the love of God, at least read up on what those drugs can do to you; even the fact that they can interfere with Coenzyme Q10 (which, ironically, is important to the heart) should be a warning that they're joke, nevermind any other side effects.
In short, I do not think that becoming vegan or vegetarian is how we're all "supposed" to eat and live. It can be an emotional or moral ideal for many; for others, it can be a health related choice; they may thrive on it because it may be what their body needs at the time, or simply because they are far more mindful on it than when they ate anything from a grocery store shelf like before.
For others, their bodies may respond much better to a "evolutionary/paleolithic/primal" diet, rich in meats and fats, including some fruits and vegetables, and eschewing grains and legumes.
Regardless, I hope that anyone who's gotten this far in my message of unplanned length read that PDF and reconsider the amount of grains they consume, as well as consider their fear (if they have one) about fat and cholesterol. I think the satisfaction that most people note when consuming a (healthy) food rich in fat says something about our needs. It's not a sugar rush - it's something deeper and more stable.
http://www.westonaprice.org/The-Benefits-of-High-Cholesterol.html
http://www.amazon.com/Fat-Cholesterol-are-Good-You/dp/919755538X
http://medicalconsumers.org/2003/06/01/cholesterol-skeptics-conference-report/
...sound like interesting places to start. Now, what you'll read is probably contrary to what you've been taught. 'Course, anyone here should know that a lot of what we've been taught as truth is complete bullshit, anyway.

Yes, there are the inevitable "skeptics" who will and do pick away at various hypotheses such as those presented above, attacking methodology or whatever else. I do find it interesting, though, that the very first site I found attacking the promotion of cholesterol admonished readers to not even think about discontinuing their cholesterol lowering drugs.

Pro-pharmaceuticals seems to be a dominant theme among most "skeptical" websites...
Ahem. My post is not to be considered professional advice. I'm not held liable for anything. These are my thoughts and opinions, observations and experiences.
No disrespect to any vegans or vegetarians intended! I eat meat right now, but I don't go shoving it in people's faces, and I equally appreciate those who choose not to eat meat not going around reprimanding everyone who chooses to do so.
Good luck to everyone!
Edit: Forgot this link, on the topic of fat; an interesting Blog called Hyperlipid:
http://high-fat-nutrition.blogspot.com/