Good question.  
I visit many boards, and cosider myself somewhat of a Board Whore also...when I have time.  (I get too anxious if I leave them for long though. LOL)  However, I definitely have the boards that I like and don't. 
For me, the biggest factor is the people.  I just can't stand to be on a board where everyone claims they're 6'7" tall and bulletproof.  Nope--I want honesty and realism.  I want to know who YOU are, not what personna that you wish to create.  I know that that GFY board has some good stuff on it from time to time, but I just can't wade through all of that crap trying to find the diamonds.  
Promotions are good for gaining people, and since I don't have my t-shirt yet, I SHOULD state that they're excellent promo tools.  However, it's been my experience that if people like the boards, they'll stay.  (Please don't dis-continue the t-shirts before I get my jersey though!!  

 )  HOWEVER, they ARE great for general advertisements.  

  I'll wear my X-biz t-shirt to a show sometimes, and branding is ALWAYS good.  
There are a few reasons why I don't visit a few boards more often though.  Some of them are because I don't like the attitudes.  Again, I prefer honest people, not trash talking and "hey--Fuck you"s.  And, as much as I hate to say it, I typically avoid boards that are Too busy.  Because there ARE a lot of boards out there, I try to visit as many as I can.  If it's too busy that I can't keep up by visiting once a day, I typically don't go there as much as I wish to.  
BUT--that's just my overworked and underpaid position!  LOL
Huggs!!!
Lady Sharlot  :-)