Number One: Volume. Live, you can only play one game at a time. Online, Much more. I've played up to 8 games, but some people can play many many more than that. As a result, you can make money even playing low stakes.
Number two: Online, people have more stats on you. They have HUDs, access to services like
sharkscope. Some people will run ICMizer on a separate machine whilst they play (not advisable). Live, the only info is what you have in front of you.
Number three: Slow play, when live, is intolerable. I will purposely go straight at someone to knock them out if they are playing slowly for no good reason. At least with online, I'll just open up another table or two if things are slow.
Number four: Online you can find any game at any stake and there will be people willing to take your money/give you money. Live, the pickings are pretty slim...unless you live in a casino town like Vegas, Atlantic City, etc.
Number Five: I've never been mugged on my way home after an online win.
Personally, I think there are far better players online than live...but my sample size is quite small.