If you're planning to play live poker, you should expect to find different types of players at different types of games. Of course, every game will have their share of nice guys and jerks, but certain types seem to gravitate more to some games than others. To wit:
7 card Stud: This game tends to consist of elderly players waiting to die.
Omaha: Omaha seems to attract the most wretched refuse of poker playing society. Omaha players always seem to be looking for a fight, whether it's with other players or the dealer, and often display some of the sourest pusses you've ever seen. A fun tactic Omaha players seem to enjoy is lying to the dealer about the strength of their hand. Omaha hands are already the toughest in poker to read, plus the dealer has to make sure that the pot is split properly, which if there are side pots and quartering, can be extremely difficult to do correctly, so for a player to announce they have a straight or a nut low when they know they don't really adds to the fun. And of course, the players will not hesitate to berate the dealer mercilessly if he/she makes any mistake.
Limit Hold'em: I've actually found limit players to be among the most pleasant of poker players. Life or death (by way of your entire stack) does not hinge on the turn of every card, so it's a much more relaxed atmosphere. These players are much happier to chat with you and enjoy playing the game.
No Limit: This game, being the most popular, attracts the biggest mix of players. There are some very nice players in the NL game, and some miserable SOBs as well. Many NL players want you to think they are professionals (generally the ones who really aren't) and a lot of them look around as if they are waiting for a bigger game, especially if they've just lost a huge pot in this one. On the other hand, this game also has players who very quietly and patiently collect everyone's chips.
Tournament: I find that a lot of tournament players have something to prove. That is, they want everyone to know that they're not the dead money at the table, somebody else is. This can be characterized by wild, inappropriate aggression or a lot of big table talk. Tournaments are also where you will find the most Internet wizards, skinny kids who are barely old enough to shave and calculate every move seemingly perfectly as they push chips into the pot and build their stacks, mostly unfazed by the occasional, inevitable bad beat.