My personal opinion so far
Graphics 9/10
The graphics are dark, moody, crisp, much like a D2 reforged. The animation is fluid, restrained and refined. Also there’s so much attention to detail, like the footsteps and the blood stains.
I hope the graphics will be steadily improved so that it will not look outdated when D4 is released.
Sound & music 9/10
The sounds effects are impressive. The music adds to the dark and moody atmosphere.
Gameplay 10/10
The gameplay looks at least as responsive as the D3 gameplay.
Seeing damage numbers in the hundreds and thousands is, to me, a big relief. I hope D4 won’t inherit the exponential damage numbers scaling from D3.
The World Events are pretty cool and add a lot of potential to D4. Would be great to see a 300 movie like mega battle as a World Event, where player cooperation has to be immaculate in order to survive a horde of tough enemies and a huge boss.
The stagger bar is a great idea.
Skills 8.5/10
The skill trees seem similar to D2. It looks like the power shifted back to a certain degree to the skill tree in comparison to D3.
Most skills I’ve seen seem to be well thought out and deliver what they promise. That said, a skill like Meteor could use a bit more “Oomph!”, i.e. a bigger area of effect would help.
Itemization ?/10
It is great to see runes like Vex and Dol make a comeback in D4.
There were are only a few unique items in the demo, so it’s only a guess, but it looks like unique items modify skills much like D3 and/or increase their power by increasing the rank of a skill like D2.
Classes 8/10
I bet there are many D2 fans out there who were wishing to see the Druid in D4. And yes, the Druid is here, and he looks great! 
Quality wise it’s at least a 9/10, but there are just 3 classes in the demo so far:
the Sorceress, the Barbarian and the Druid. I’m hoping to see an Amazon, Paladin, Assassin and Necromancer class added in the near future.
Conclusion
Treasure Goblins, random events, World Events, unique items, Mythic items, dungeons of different levels, lots of skills to chose from. I feel D4 has a good game core now, but it’s too early to say anything about D4’s depth and endgame.