hrm, I'm not going to touch on the classroom stuff, that's not my area of expertise, and those that have taught in Thai classrooms can share experiences. If you can embrace the attitude that only things involving 'sanuk' are worth doing, then you may like Thailand after all.
The visa situation in the Kingdom is constantly evolving. What I'm writing is true NOW, but it may not be true tomorrow and it might have changed since you were last there. I know this to be the current situation though....
1st... visa exempt countries.
There is a long list that includes most western nations for visa free entry to the kingdom. These are generally referred to as "stamps" within the kingdom. They get you a 30 day stay free of any paperwork when you arrive in Thailand by air. If you cross a land border, you are given 15 days. These are given to anyone. You can enter as many times as you like in a year on these "stamps," but are limited to stay only 90 out of every 180
consecutive days. There is no extension for visa exemption stamps.
Tourist Visas
T class visas are given at any Thai embassy or consulate, anywhere in the world. They are valid for
six months from the date of issue. They are generally given for periods of 60 days, and another 30 days can be added at immigration offices within the country. When you are in the country on Tourist visas, your time DOES NOT COUNT for you 90 day limit on exempt stamps. Tourist visas are not a viable way to stay in the country long-term, as once you have so many in your passport they will decide to deny your visa and add a stamp to your passport letting everyone know you were denied a tourist visa, and also letting everyone know you are now banned from the embassy where you applied (this happens a lot in Penang).
Education Visas
Education visas are difficult to come by these days. It used to be very easy to enroll in Thai language school and get your paperwork, and never show up for a course. These days are now gone, there is no easy ED visa anymore. Thai language schools are now very tightly controlled on their visa sponsorships, and if you miss a class at language school you may just have your visa revoked and end up persona non grata in the kingdom. They really will do this, as I have seen it. They don't even tell you your visa has been revoked, they just do it. The schools are required to report your attendance and progress, not something I'm willing to mess with.
Business visas
Type B visas are generally very easy to get, so long as you are willing to dump 800,000 THB into a Thai account (where it's very difficult to expatriate the money again) and are willing to apply for your visa from your home country. Note that this does not give you license to WORK, this is not a work permit. These visa come in a variety of lengths and extendabilities. Any good immigration attorney can help you with one. You will need to set up a sham company, have Thai board members of said company, and also show assets in the kingdom of a certain value, aside from your bank account.
Type O Visas
Type O, or non-immigrant visas, are probably the most common visas for long term expatriates of the kingdom. The only way you will be issued a Type O without the right help these days is by being hired by a Thai institution. Foreign teachers are now required to have a TEACHING DEGREE, not just a random 4 year degree, to teach legally in the kingdom. So unless you have a teaching certificate from your home country, you will be working illegally (or willing to pay outrageous fees for a Thai certificate). These visas are sometimes available anywhere in the world, at any Thai Embassy or Consulate. Availability changes, some months they need to be applied to from your home country, others any Embassy or Consulate will do. They require documents form your employer, a background check from your home country (even if you haven't lived in or been to your home country for many years), and the usual pointless bureaucratic forms.
WORK PERMITS are separate from visas. A business visa /=/ a work permit, and having a work permit with a B visa does not entitle you to work. See, it gets complex

You must have the correct visa with your work permit for it to be valid. Work permits are tied to an employer and a location. If you worked for a private language institute which had two locations, you are only legally allowed to work at one location, the address displayed on the permit. You are working illegally if you are caught at your employer's second location. Work permits are quite spendy, and are designed to discourage hiring foreign workers. In some areas, monthly taxes on foreign workers run 8000 THB a month, the same as many Thai's salary.
Hope this helps. It's daunting, I know. But I managed to hack it for three years in the kingdom without going crazy with visa nightmares. You'll find a way if you want to stay.
One more note, overstaying is no longer an option in the Kingdom, it's become prohibitively expensive and the Thai's are not shy of locking up westerners for immigration violations. It's now 500THB per day,maxing out at 20,000 penalty. You will also do your jailtime if you overstay too long, or are caught on the street with expired papers. If you make it to the border, and are on the way out, but cannot pay, you also get to visit prison.