Off topic I know, but since you clearly know the game well, what are your favorite classes?
I know I wasn't asked, but I'll share my experiences. I started out playing as Zer0 the Assassin. It was alright up until level 20 or so, and probably about halfway through the story. (I have a tendency to do a lot of side missions and grind a bit) Around that time, his special ability kind of became useless. Maybe it was because I didn't follow the right paths in his skill trees.
I decided to start over as Axton the Commando. My view is that his ability is the most beneficial, especially to a solo player. His skill tree also lends itself well to boosting HP and Shield stats, and lots of HP and Shields isn't a bad thing. Throwing down his Turret can get you out of a lot of situations where you're beset by a bunch of enemies, and if you put in the skill points it can create a shield barrier which can protect you.
I haven't tried the Siren or Gunzerker, and don't own the DLC for Mechromancer and Psycho. Playing online you'll see numerous Mechros though, and their attacking robot ghost thing can be pretty useful. I don't know their skill tree though. You'll see some Sirens, and to me they seem like a support type character. They're a bit more beneficial to a group as opposed to going it solo, in my opinion. I have no first hand experience though.
I really don't know much about the Psycho. I tend to hate any I come across online. Their whole thing is to be crazy, and rushing around doing massive melee damage. I've found they tend to die quickly in a large firefight, and this can be annoying. It would be better to have someone who's got average weapons but can stay alive instead of someone more focused on one enemy who may die a lot and you have to wait for them to get back to the fight.
That's all just my two cents based on my experiences, and also from playing on PS3 though I don't think PC would be much different if at all.