Blinds go up faster, buy ins usually aren't as big, and you get dealt a lot more hands in an hour than you would see live. Oh and poker sites give you ~1 minute max to make a decision, in a live tournament you can sit there for 5 minutes waiting for someone to decide.
^^ this.
Probably one other thing too - it depends on the game and the venue, but quite often live chip stacks are deeper than online tournaments.
Take the Main Event of this year's
wsop: two-hour levels, 20,000 starting stacks, and the initial blinds were only 50-100.
In the average online tournament, you might be starting with: 10 minute levels, 1500 stating stacks and initial blinds of 15-30.
That's a 200BB stack vs a a 50BB stack.
That said, there are some online tournaments that go for quite some time. Didn't the main event of the WCOOP at Stars run for like 19 hours the other week?