Just my 2 cents and my thoughts when I first read the headline and OP (apologies, I'm in a rush and can't read all the replies).
The only thing that worries me about a general forum and open application forum is that I think it's important that a person is not just apping up to every forum just to be a mod, but the person is interested in the forum in which they want to mod. I can only speak for SLR, but we usually see people we really like and want them to mod. Noodle is much more aggressive with invitations (which is one reason we love him :D), but we do take notice of people who generally want to help, have some compassion and are a good fit for the SLR model. Not everyone (similar to TDS) has the ability to be supportive while still trying to help in a straight-forward way. I wouldn't call it "hard," but I think a certain type of person is needed to mod the support groups where people just really need a shoulder to cry on.
I think it should be mentioned that some forums require a history in that forum, because some (MOST, IMO) forums require you to understand the dynamics and idiosyncrasies of that forum.