I've just moved up to 400 NL - here is what I've noticed.
More aggression (duh). As in more check raises with draws, more pot C-bets with air, etc...
A lot more "loose" players who profit playing that style. At 200NL green numbers in Holdem Manager equited to donks usually, but not as much here.
I've found a lot more bad players than in 200/NL - guys who you can look up the stats for and say "man - how does this guy afford to continue to play?". Small sample, but just what I've noticed.
The biggest problem with moving up is that you don't know who the regulars & decent players are without hitting the ratings to see who plays a lot and who doesn't. I've averaged 3 BB/100 200NL for months now because I recognized the regulars and knew how to beat them. Move up, and you start all over.