This game, pending on an actual release date is going to be pretty stellar.
Graphics: It's smooth and clear, definitely leaps and bounds ahead of d2 which is expected. There are some kinks and what not still with the beta and I'm hoping that they'll be polished up by the time it's released. Don't expect to be blown completely out of the water if this is your first time playing the series, but if you've played diablo 1, Hellfire, 2, and LOD you'll be astounded at how much better the game looks and feels.
Gameplay: For anyone who hasn't ever played an mmo it might be a little strange to play with the top row keys but it's easy to get down. I've spoiled myself with peripherals and have a razer naga that I use so about the only thing I use my left hand for is holding down shift when I want to do standing attacks.
I really enjoy the facets of using the different abilities. A great example of this is the witch doctor. I'm able to be putting up a dot, throwing down an aoe, as well as throw frogs at people that explode in poisonous fury. It really has that mmo feel but I think Blizzard was able to get it right by limiting the amount of abilities you can use. I know some mmo's feel very very overwhelming with the different abilities so keeping it simple is much better. Add on the fact you can use the runestones to change up your entire skill set it just adds a whole different level to the game overall.
The auction house is still underwork so there are functions that aren't useable but once those are ironed out I really feel that the auction house will be nice addition. I don't have much of an opinion on the RMAH itself but to each their own. I personally don't plan on using it.
The game overall feels much smoother and way more polished than the previous games. Some of the tedious things from d2 are gone, like talent trees and point allocation with your stats. I find this a god send. Nothing was more boring in d2 than, enough strength for gear, max block, and rest into vitality.
Two words; Shared Stash. Huge amounts of space and it's shared throughout all the characters as well as gold. This is a freaking god send. I've lost many a rune in diablo2 due to muling. No longer will that happen.
My one big irk I've found so far with the gameplay is I can't tell if namelocking works. Maybe I'm not watching it enough but I swear sometimes it works and sometimes it doesn't.
Replay Value: Oh so much! With five very different and very fun classes I can't imagine not leveling all of them to 60. So far in the beta it hasn't felt like a chore so hopefully that trend will continue!
Overall Opinion: The game is going to be fun. I hope a release date comes soon because I can see myself putting a lot of time into this game just because of the shear enjoyment I'll get from it. I've been playing diablo 2 off and on for the last 9 years so I think its high time the game comes out and wows everyone.