Played through as Wizard twice, and Demon Hunter, Barb, and Witch Doctor once. Will be doing Monk tonight.
I was really surprised with how fun witch doctor was. It was one of my lower "interest" classes going in but I think it's going to be my first alt when the game's released. Haunt + Grasp of the Dead = absolutely awesome. Zombie dogs are nice for keeping things in place/taking damage. Toads seemed a bit underwhelming and poison dart while nice, just doesn't look/feel as awesome as some of the other skills. Firebats works, but my playstyle (as WD anyway) suited Haunt + Grasp of the dead moreso.
Barbarian is great if you like smashing people in the face as bodies fly across the screen. Quite powerful with a nice 2 hander + Cleave. Frenzy is a lot of fun too, absolutely tore up the Skelly King. Feels like a really powerful class. (then again, every class seemed quite powerful).
As for the Wizard, I was quite surprised with how awesome spectral blade was once I had a nice damage weapon. Arcane orb is powerful but eats mana fast. Electrocute is also just sick in terms of damage/clearing groups. Played with magic missile a bit but it seemed underwhelming, but it's still decent. Shock Pulse pales in comparison to Electrocute. Will be playing this char first.
Demon Hunter is pretty entertaining to play when incorporating Evasive Fire/Vault and backflipping all over the screen. Bola shot is quite good once you get a good hand crossbow, as is chakram. Bola shot is nice cause you can fire multiple at a time and have it chain explode off the same or multiple mobs. I didn't think chakram would be as good as it was, but you can easily clear a room full of mobs by spamming it a little. The hatred cost of chakram seems low, but I approve

. Rapid fire (shooting at 6times normal speed) feels like you're firing an automatic gun at the enemy - lol. It's pretty good though in terms of the "feel" and effectiveness of the skill but burns hatred quick.
Overall, it seems every class can hold their own/wreck face and I know I'll have every class to 60 at some point. Looking forward to trying Monk tonight. I watched my buddy beat the beta as Monk and he seemed to enjoy it quite a bit.