First, my credentials:
I am a massive Diablo fan who has been playing the games since D1, I actually played D1 online. I played D2 off and on, noobstyle and hardcore for years. All my friends played D2 fanatically as well. I even botted for a while at the end of my D2 career, so I know the game inside and out from every side.
So lets get right into it.
STORY
We'll start off with probably the least important aspect of the game, which is good, because I don't know what the fuck happened to the writing staff at Blizzard over the last few years. Blizz used to do a pretty damn good job with this, go play SCBW, it didnt just seem good because you were 13, it's actually well done. In this department they pulled basically an SC2 on the Diablo franchise. The story is fucking horrible, the dialogue is bizarrely inhuman, and the complexity level is sitting around 3rd grade "epic" fantasy. Basically demons are evil (in a totally boring and unsophisticated "I hate puppies muhahahaha" way", and you have to kill them. There is a twist that nobody cares about because the characters have the depth of a kiddie pool someone left uncovered in the Sahara desert. Its just bad, simplistic, and uncompelling. Fuck the story.
+ Show Spoiler +
"Uncle Deckard, why am I so fucking boring?"
KILLING DA MONSTERS
Here's a slightly more important aspect, and its one that Blizzard has really done an excellent job with. If you played D2 you know that basically all you end up doing is very basic kiting and spamming of a single skill 99% of the time, which is certainly not going to work in this game. They really added a ton of different enemies and abilities into the game, which combined with the new skillbar system give the combat MASSIVELY more depth than any of the previous games. In order to be successful you will end up using all six available skills frequently and adapting to your enemies.
SKILLS
The skills are very interesting (mostly), and trying out new builds is fun. It does seem there are alot of skills that are just flat out shitty, but I guess thats OK. The utility skills in particular are very useful and cool, and the passive system is done very slickly and will lead to lots of odd builds later on when people figure them out. Here's an issue though, you unlock skills every level, and then later on you will get runes to add or change their effects. The problem is, there is absolutely no advantage to using a skill with no rune, in fact, the runes are a direct upgrade to the damage in nearly all cases. So even though you just unlocked sweet new Firebombs, you really can't use the skill until you get at least a rune for it, as any other skill you got earlier in the game and have a rune for will be automatically better.
ENEMIES/BOSSES
Like I said, the combat is just 4000% more entertaining than D2. The bizarre part is when it comes to difficulty. They have added all sorts of modifiers that can be randomly added onto champion level mobs around the map, but once you get into Nightmare mode and further, these frequently combine to form monsters that would tear any Act bosses face off and leave you crying on the floor with a bleeding anus. The bosses all have unique attack patterns that you can learn and all sorts of skills to dodge, but them seem to be well balanced to take a certain measure of skill before going down. These random champion mobs often end up absolutely unstoppable and occasionally forced me to leave and start a new game because I was completely unable to dent their health bar while they jumped all over my face with lazers and super speed and vampirism. This is fucking annoying and pretty shitty game design if you ask me. I'm not bad at games, I'm not going to sit and farm items for 20 hours to be able to kill these occasional uber mobs when I can easily handle every other boss and enemy in the game. Hell, I'm not even sure there IS a gear level sufficient to handle some of the more imbalanced combinations. Enemies with abilities that do basically infinite damage that you are supposed to dodge, while at the same time also immobilizing you, just pray they don't do both at the same time.
MULTIPLAYER
This is a wierd one. They have a great friend system that allows you to basically instantly drop into any of your buddies games. But in my experience the game actually becomes significantly harder when playing with more people as enemies get huge buffs to health and damage. The only reason you would really want to party up from a gameplay perspective is because of massive use/abuse of the resurrection system. I die like twice as much in a party, we only keep going because everyone keeps ressing each other. You can watch the dedicated clans playing Inferno and its an absolute orgy of rebirth that would put Jesus himself to shame. Since dying isnt really fun for me, this actually makes me want to play solo, to the extent where I boot my friend out of the game in order to kill hard bosses...which is really not what I want to be doing.
ITEMS
Diablo IS items. This is a category I'm still on the ropes with. On one hand I feel there has been significant simplification of the item system, with basically everyone stacking their primary stat and health, but on the other hand as you progress up the difficulty levels they really start adding more modifiers. I'll refrain from true judgment until I beat Hell etc...but the impression I'm getting is that I feel a lot less that I'm customizing my character, and more like I'm getting "better" shit to make my DPS number increase. The system with every skill based on weapon damage is not very satisfying to me. They claimed they would have lots of systems in place to prevent wizards from running around with the biggest axe they can find, but that's exactly what I have been seeing, and it also negatively impacts the feel of customization and having a unique character. In D2 you felt cool having a character all decked out to have a really specific purpose. You were a badass unlike your buddy Jim, because you had a level 35 Meteor with all your plus skills gear. These specific skill damage boosts seem to be extremely rare in D3, which sucks because instead of having some kind of identity to your hero (#1 HolyBoltadin NA), every Wizard is basically identical to every other Wizard, except one of them is better. Maybe these specific things will appear more later on...but i shouldn't have to wait until 200 hours into the game to make a unique build.
A really strange aspect of the game is the fact that white items are, as far as I can tell, utterly worthless in every possible way. In Diablo 2 you would still pick up and be constantly on the watch for a really good socketed and/or superior white, which could be imbued, or more likely used in high end runewords. A really good white could be worth more than nearly any unique. In D3 there is literally no reason to ever even pick up a white item past the first hour of gameplay. There are no runewords, and you cant use them in crafting or any sort of imbuing, even the best one will only sell for like twenty gold. Why are they even in the game? When you take this into account you realize that the amount of items actually dropping is very low compared to D2.
MENUS AND GUI
Ok, inventory and shit are all fine. Item comparison is very nice, but thats all been standard for years. I have beef with the skill trees.
What the fuck is this.
1. There are THREE nested menus here to navigate: The left and right arrows, THEN you have to click the skill to view its runes, THEN you have to mouse over the runes to see them.
This must be because there is a lot of information to display and not enough space right? WTF NO.
2. Look at how this page is layed out, the fucking enormous retard buttons and 90% empty space. Why? There is absolutely no need. All of the rune information could easily fit on one screen. Why is there no indication of which rune you are using for each skill unless you select that specific skill?
Why are the skills even put in these wierd categories with left and right arrows instead of simple tabs so we can see where they fucking are? How would you like it if your browser looked like this?
+ Show Spoiler +
You wouldn't like it because its terrible design. They dont even give you the most basic features to make the menu usable. You remember these from your Iphone?
+ Show Spoiler +
They are there to give you some sort of reference point so you know where the hell you are in the menu. Can we get some of these? The current system is like finding your spot in a book by randomly flipping pages one at a time until you see something you recognize.
Like I said, why are the skills even grouped in these arbitrary categories anyway? This goes into the whole bullshit that is elective mode. They really think players are going to be confused and pick the "wrong" skills...really? How are these categories in any way helpful to the player, they aren't even consistant. Like here's a category called "Decay"
How is this supposed to help me? Am I supposed to see a skill and be like: "Oh, that will be under the Decay header obviously". Fuck no, these skills are completely random in every way. We have a short range AOE poison attack, a long range single target spirit attack, a long range AOE poison attack, and for some random ass reason a defensive utility wall...They arent even internally consistant. What the fuck does Spirit Barrage have to do with decay? Wouldn't Zombie Dogs or Locust Swarm make a hell of a lot more sense? This category is completely worthless. It might as well be called "Lippo Jungies" for all the organization it provides
This skill menu sucks ass, and it pisses me off every time I level up and have to search through it for 10 minutes trying to find what I just unlocked.
CONCLUSION
Ok, despite my flaming about certain aspects, the game is good. The gameplay in itself is massively deeper and more polished than any other similar RPG. The heroes and skills are great and creative. The game is fun, it just has a few very specific issues. The only thing I'm really worried about is the item system...I hope they take it in a more complex and customization direction instead of continuing to "streamline" it. Only time will tell. The challenge level is very high and its refreshing to play a game where you will actually die several times before accomplishing something in single player.
Definitely worth a play, looking forward to PvP, which will probably be very fun.