First the game plays really smooth. I have yet to encounter any bugs or crashes. The UI seems simple yet functional and is easy to use. The various systems such as joining matches, the auction house, and the achievements all seem to work really well.
The abilities system is just amazing. It is a great step forward from where they were in Diablo 2 (I am a huge D2 fan and have played since its release way back when). The use of abilities feels so fluid. You quickly are able to match complimentary abilities together to maximize your hero class and playing style. Discounting the fact that we don't have the rune system and that we don't have full access to all the skills and the entire skill bar the system in the beta still feels like a huge advancement beyond what Diablo 2 had. The abilities system is amazing now, upon release it will be epic.
That said without having the rune system in Diablo 3; the game would still be fantastic. It would far exceed the expectations of its most avid fans (of which I consider myself one) and would be a deep and variable experience. With the rune stones implemented I can't even fathom how incredible this game will be.
The atmosphere and physics of the game are great. Walking through Old Tristram feels dark and spooky; knocking undead through the air only to have their half destroyed bodies crawling back at you while their entrails hang out is awesome to behold.
The music so far fits the situations and is a great compliment to the game's visuals.
I don't think the game feels cartoony or wowish. The art style feels true to what Diablo 1 and 2 were, but with the added enhancements of our modern day technologies. Diablo 3 contains greater detail and a darker tone than wow could ever hope to achieve. There is a grittiness and harshness to the game that comes from the art style, Diablo 3 will be no kiddies game.
Diablo 3 feels very polished to me. If they released this game tomorrow I think it would be the game of the year. With the further work and enhancement coming this game will be amazing for years to come.
So having played the beta I'm not worried about who works or doesn't work at Blizzard on this game, I'm not worried about what changes they are making or what systems are getting overhauled. In an age of skyrim and other buggy launches It is nice change to see a studio like Blizzard work and polish a game to increase the product quality from day one.
So don't worry Diablo 3 is currently great, but will be upon release amazing.