I was invited to D3 beta a few days ago and I've been playing it since then. I have played on all classes. Here are my impressions so far.
Positives:
I liked the graphics and colors. The UI seems to be fine. I liked the fact that the gold you're making is accessible by all your toons. You do not have a different gold account for each toon. Likewise all your toons access the same stash (your bank in D3). So, if you find a Wizard unique item while playing your Barbarian you can put it in the stash and retrieve it when you are playing your Wizard. I don't know if this will be the same for all items in the game or if there will be items you cannot pass between your toons.
Negatives:
The talent system looks a little too simple and restrictive, however it may work fine for Diablo.
Unfortunately, Diablo 3 is dumbed down the way WoW has been dumbed down. Apparently, Blizzard does not think challenging game play is the future.
The game holds your hand very very tightly. Here is how questing works:
1 - Game tells you to use the waypoint to go to Cathedral grounds (and shows where it is on the map). You go there.
2 - It tells you to go to the Cathedral gate (and shows where it is on the map). You go there.
3 - It tells you to enter the Cathedral. You enter.
4 - It tells you to go to Cathedral level 2 (and shows where it is on the map). You go there.
So on so forth.
There is no exploration or discovering or finding something. Sure you can ignore the quest instructions and go into the wilderness despite knowing the Cathedral's gate is not there. However, this won't be the same fun you'd have by exploring and discovering the quest objectives on your own.
Mobs are way too easy to kill. I literally yawned and clicked my way most of the time. I tried multiplayer and it gets even worse. All everybody does is trying to run ahead of others and button mashing.
In short, if you like the way WoW has been dumbed down, if you are comfortable with the game holding your hand tightly, if you are not interested in challenging and admittedly time consuming gameplay then Diablo 3 may work just fine for you.
If you have problem with these attributes and you are not happy how WoW has evolved then you may want to find a way to check out Diablo 3 before buying. Try to find a friend who will buy it for sure and play his Diablo 3 before making your decision. It may save you $60.