Disclaimer: I don't belong to any sort of chess club, so OTB might mean something a little different for me and others like me.

One difference for me is the time constraints - OTB games for me are quite casual, and I don't own a chess clock, so while the players might rib each other during a particularly long "think", there aren't any hard and fast time rules.  On the other hand, internet games always have the clock running, for obvious reasons.  My play is worse online, probably because of that.  For one thing, I never have the time to set aside an hour for an internet game, so I tend to choose the 10 min ones, resulting in shallower play.  My OTB games, on the other hand, tend to go a lot deeper.

I haven't noticed any objective effect on my OTB games from playing on the internet, though.  Since going online, I've been able to appreciate the lack of pressure in casual OTB games.  The net effect is that I feel I play better OTB, so I'm more confident than I used to be.