A short summary of each class:
I started with the wizard, as it is the class I want to roll at release. Fantastic, fun, and engaging. Resource system worked for me, and I had fun. I like to shoot spells, I like to devastate things, I like to generally be a caster. It was everything I liked. Was not let down in the slightest. Particularly enjoyed disentegrate. I will definitely be rolling a wizard on release, and I will definitely be using my current favorite skill.
I tried the Witch Doctor, and it was so fun that I almost considered rerolling away from Wizard. I think long term I will prefer the resource system style of the wizard more than having to manage the mana of witch doctor and utilize a few summons, so in the end I will be rolling a Wizard first, but witch doctor will most certainly be my first alt.
Barbarian was next. He was immensely fun. I liked the barbarian in D2 and I like him again. Not my primary style, but a good time spender. Fury generation is easy and intuitive, and spending it has good effects. I'll enjoy it even more when I have a more diverse skillset available. Did not spend too much time.
Demon Hunter was next. Fun. Overall higher "skill cap" even in the insanely easy beta. As a very familiar Diablo player and gamer, I shattered beta content even on my first playthrough. I didn't die, but Demon Hunter was the only class that I even remotely came close to dying. The reason wasn't so much that the demon hunter is weaker, but rather that the weapon choice is restrictive, which isn't true of any other class. Since I could only use bows or crossbows and the like if I wanted my skills to work, I had an unlucky run of drops so I had to get by with really bad DPS. Once I got a decent weapon (crafted at level 6), it was easy as pie again.
I didn't try the monk.
Content wise : the beta was great. It is way too short, but I understand why. It's more of a teaser. If the love and care that was put into the first half or first third of act 1 is anything like the care that was put into every area of the game, then the entirety of the game will be fantastic. I am more excited and want D3 more than ever. It is fun in the exact way that I want it to be fun. I also cannot wait for much more challenging difficulty levels.
As far as criticisms and complaints I have one very large complaint. Simple tooltips are really, really poor. In the midst of a vast sea of compliments and enjoyment, I really do not like not being able to see at least what % of my DPS the spells or abilities are applying. It's a really poor choice. Yes, I like to min-max. Yes, I will be using an outside resource to figure out how to mold my play style. Yes, it is really annoying. I want to be able to access this information in game. I realize that my one voice in a sea of voices is very small, but please consider it.
New game changes : No problems, salvaging, selling, inventory management, town portal, identification, all not a big deal. Very easy, very intuitive, very much geared to letting me get back to blowing up monsters. Also like the removal of the skill change altar.
TLDR: Classes good/fun, beta content good/fun, polish nice, overall "A", SIMPLE TOOL TIPS BURN IN A FIRE.