People are generally tighter at 100nl, you still get a fair number of fish, compared to mostly all fish in 25nl and stuff. People are much more aggressive there, and the players are definately much better at 100nl. Hands like flush draws go down in value, because alot of players know to cut off your odds, and can fold when the flush card hits. Concealed hands like trips and pocket pairs go up in value because of peoples aggression, and inability to figure out what you have.

I've actually found pokerstars 100nl to be relatively fishy, and party poker to be filled with decent players. Although i haven't logged alot of hands on those two sites.