I play about 95% online and 5% live. It used to be 100% live, back in my younger days. I enjoy playing live more, but overall I have more success online.
To me there are a few significant differences between the games, in addition to pace of play, bet sizing and potential distractions as already mentioned:
(1) Tells. About the only types of tells you can get online are bet sizing and timing. Live, you can see the other person's face (are they looking at their chips? Are they looking at the board? Are they looking squarely at you, trying to get a read?), hands (did they stop playing with their chips when the board hit? Did they bet more deliberately, counting chips carefully, or just shove chips in?), overall posture (do they look defeated? Confident? Are they overacting?), etc. It's much easier to pick up on tells in a live game. Inversely, of course, you're much more likely to give away such information in a live game than you are online.
(2) Rake/toke. If you play SNGs or MTTs online you're used to about a 10% tournament rake regardless of stake (smaller for turbos or special tourneys). A lot of live daily/weekly/monthly tournaments not only have a higher rake, but if you win you're expected to tip the dealer as well. Same for cash games; rake can be higher, especially if there is a bad beat jackpot or high hand jackpot, plus, again, you're expected to tip the dealer.
(3) Etiquette/card handling. Accidental things like betting out of turn, revealing your hand, folding prematurely, etc., can happen live (and, depending on the circumstances, can mean a penalty, like having to sit out for fifteen minutes or one round of blinds). Once, I accidentally mucked the winning hand at showdown. That can't happen online. On the other hand, I have, on occasion, been flashed a card or two by someone sitting next to me (I don't look on purpose, of course, but if they're holding their hand so far away from their body that I can see it, it's not my fault), and while it's good form to inform someone when they are doing that, you can use whatever information you can get to determine your action.
(4) Convenience. The nearest casino is an hour away. My computer is between the kitchen and the living room. And, I get my own parking space.
(5) Chips. Online, it's just pixels on your screen. Live, you get to play with your chips. I know, this sounds silly, but to me there's nothing like feeling and hearing the clattering of chips as I stack them or shuffle them. Or try to launch them tiddly-winks style across the table into my brother's beer (it was a one-in-a-million shot!).