Mandatory "I was a hardcore D2 player and still play occasionally and love it and I played in the open beta weekend and here's my opinion guys listen k" intro.
To me, the most enjoyable skills in D2 were the novelty ones. Unfortunately, they usually weren't strong enough to be viable beyond Nightmare (if you were lucky). The assassins traps are good examples. A blade sentinel the spins back and forth chopping everything it touches? Sweet. Firebombs? Great. Charged bolt traps? Loved it. However only a few of those worked well and only with syngeries. Most people just invested in them to get to Death Sentry. You COULD make it through the game with most, but it was a real struggle, made fun by the novelty of the skills (I found, anyhow).
That being said, some of the stuff available even in just the first 13 levels of these D3 classes is fantastic. The demon hunter's chakrum (or whatever) is a blast to use. The monks first few melee skills already top anything the assassin had in terms of her kicks and punches. The wizards frost beam is much more enjoyable in use than D2's ICE BOLT ICE BOLT ICE BOLT ICE BOLT. Hell, they even managed to make the barbarian fun.
We can go on forever before release (and probably after) about how one game's system is better than the other. You can never satisfy everyone. However, the fact that I'll be able to go from level 3 (4?) to 60 on a witch doctor and throw jars of spiders (!!) the entire time, AND have it be viable throughout tells me that for me, Blizzard has succeeded in making something that, comparisons aside, is just plain FUN.
Seriously, jars of spiders.