For me, at least, Diablo 3 has been on a down hill slide ever since it was announced. I am a core fan of Diablos 1, 2, and LoD. I read the lore in the manuals, I stopped at every book in the cathedral, I tried to hear what every NPC had to say about everything because it was a fascinatingly dark world. The art style was Gothic and pulled no punches, and neither did the story. Amazingly, there was an interesting character development behind strikingly simple game play as well. This character development was complicated a hundred times and made far more engrossing in D2, and in many ways created the ideal complexity that was necessary as the basis for such simple active mechanics. Hell, I still remember toying with strange character builds - the dagger melee necro, the java-barb, the caster Pally. This was possible (and in some cases remarkably successful) because of how open D2's customization was. All core stats were alterable, which meant you could forgo the melee centric Strength and Endurance on a Barbarian to try something unusual, or dump great deals into mana for characters who traditionally don't need that much. It was brilliant. Nay, it was genius!
And Diablo 3 has taken every one of those points and changed them into something simpler, and in some cases just gotten rid of them all together.
lore
*Diablo 3 screwed up the lore with its first trailer, and it has gotten progressively worse as time has gone on. This makes sense, as Blizzard publicity admits to having no regard for their own properties. "Game play first." Its their development policy. Warcraft has ballistic lore because of this. A game or expansion in which a character/species' entire motivation isn't completely changed, or a major event isn't retconed for the convenience of a new 'level' is unheard of. Spoiler alert, Medivh didn't open the Dark Portal! They decided he did in Warcraft 3. Did starcraft 2's plot seem a little strange with the advent of magic and sudden change of heart for the Overmind? It was. Blizzard was making the entire plot up with each level. So far in Diablo 3, we know that Tyrael is evil/dead (because how shocking would that be), Deckard Cain has an apprentice that can see the future (Despite swearing the knowledge of the Horadrim would die with him), and Tristram was turned into a tourist trap (because seeing the heart of evil in fun! Bring the kids!). That was just in the first 5 minutes of the game's announcement. It has gotten worse.
I am curious as to what you mean by changing the Dark Wanderer's back story, though, Pothb. Is he no longer the warrior from Diablo 1? Did Blizzard nonchalantly change that too?
art style
*It was a point of remarkable debate early on as to whether or not Blizzard was actually changing the art style. People on both sides of the argument swore up and down about it. Now, anyone who thinks they didn't change it arbitrarily has shut up. In the first demo, there were, notoriously, rainbows and butterfly's. The color palette had been expanded greatly, the tonal lighting had been abandoned for a more flat filtered look as well. But what stuck out like mad to me was the character design. It, for some unknowable reason, maintained the same exaggerated proportions and details that Warcraft 3/WoW did. Then they showed off the second class: A Witch Doctor. A Witch Doctor. To make it even better, he even looked like one of the WoW troll variety (although the D3 version's mask was comically much, much larger). Then the wizard, the monk and the Demon Hunter. Wait, what? A Demon Hunter? Oh god, the only thing that could make that idea worse would be if it looked like some damn night elf with glowey eyes... oh WTF?! Its no secret that Blizzard Irvine is behind D3. Its also apparently no secret that Blizzard Irvine's art staff can't accomplish art styles beyond their own exaggerated WoW, high fantasy visuals. Or maybe, Blizzard Irvine, so convinced of the pure god like master work WoW's perfection, decided to make everything look like it to draw in the WoW crowd. Well, its working. I was on a date with a girl recently who informed me that she "Sequee'd" (a term used to describe an excited, barely contained exclamation of joy) at the Demon Hunter's appearance, as she looked like a night elf/blood elf. Unsurprisingly, this girl never played Diablo 1 nor 2. Blizzard has made some small efforts to reduce how ridiculous its characters look, but not much. It still looks like Iso-WoW with more filters on.
But lets be honest. All of that could have been excused if it weren't for this next part.
Gameplay and mechanics.
*How you can screw this up any worse without intentionally trying to destroy the game is beyond me.
First, the removal of potions (although they have been reinstated in WoW limited use form). Instead, now creatures drop little blue and red orbs ala God of War that when absorbed give back health or mana. This change was made as Blizzard felt it "would keep the players more in action, instead of having to retreat to use a potion." I have a hard time imagining this to be true.
Second: Slowed game play. D3 has stronger mobs, some even requiring "strategy," like the shield skeletons who can't be harmed from the front, or the numerous other mobs who take an inordinate amount of time to kill in the other videos. While I'm not inherently against this, this move becomes dangerous to the ARPG formula which lives and dies on its pacing. Run speed is a very important part of the ARPG formula because it dictates how quickly you can traverse the world and engage enemies. The core, simple game play mechanics of the ARPG require fast action in compensation. No one likes how slow D1 was compared to D2 because the speed and frantic pace of D2 lent itself far, far better to the simplicity of the combat. The stronger the mobs are, or the longer they take to kill because of "strategy," the more you slow that frantic pace until your only killing a handful of enemies every minute (rather than the hundreds that could be accomplished in D2). This, of course also damages your acquisition of loot, another extremely important part of ARPGs. If mobs take longer, then there will likely be fewer of them. The fewer the mobs, the lower the loot acquisition rate, the more tedious the simple, repetitive gameplay becomes. In addition to this, the Isometric angle is closer to the action, making it look cooler, no doubt, but this (oddly enough) actually demands slower character movement because the player can no longer see where (s)he is going and in order for the more detailed environment to render and be visually appealing, it must also be introduced slower. This slowed game play is actually one of my problems with Titan Quest. Its a great ARPG no doubt, but it gets boring quickly because the characters move much slower, attack much slower, and individual mobs have more health, which means the experience just drags a great deal.
Third the AH. This is a cash grab by Blizzard. They want to get in on the gear exchange market so that they can get their cut. Were it that simple, this wouldn't be that big of a deal, but it isn't. What it does is open up the possibility (not a guarantee) for Blizzard to manipulate their game to make them more money and put the titanic mountain of money they make from WoW to shame. People are bothered by this and they should be. As was mentioned previously, Blizzard can alter drop rates on class specific (favored) gear so that those classes will only receive other classes gear. What can exist under the guise of collaboration is actually Blizzard using the eastern ftp MMO business model of give us money and you get good stuff, or as it is commonly known, pay-to-win. Best of all, if you want to cash out on the money that you "earned," you have to do so through a 3rd party organization who get to claim an additional percentage of all transactions, so its in your best interest to keep your money invested in Blizzard so that they get to keep it all anyway.
Fourth, and this is the king of them all: automation and simplified core mechanics. Diablo's active mechanics are not complicated. In fact, they are incredibly simple. On their own, they would make for a very dull game. What complicated Diablo (or any ARPGs) is the mechanics beneath, the base upon which the combat is built. Items, Skills, and Stats. The ultimate Trinity of character customization. ARPGs are the best in all the world at these three things. ARPGs live and die on thes three things. Diablo 3, wants to eliminate two of them for the sake of convenience. The initial announcement of character stats automatically leveling (Ala WoW), was bad. In fact, it was borderline devastating. Those core stats are the building blocks of every character. They define who and what your character is becoming. In many ways they define what roles are available. *My single largest complaint with Torchlight is that Health and Mana both advance based on character class, not player input. What this meant, is that my vanquisher never had enough health to be awesome in melee, despite having skills that were great for it. It meant that if I wanted to have an effective Vanquisher, I had to play a gun totter. Diablo 3 took this one step further and decide that all characters of a class, are basically identical because its more "fun." They then took it even further and decided that all characters should be identical in higher mechanics too by automating skills gain and advancement. Granted, D3 does give each active skill 3 "slots" that can be used to customize that ability to the player's liking... but in all but tweaks, all characters are identical. No matter how much you may want to experiment with building a class to do something unusual or new, like a java-barb, caster Pally, or melee necro... they will never have the stats to support anything but what Blizzard designed them to support. There is no freedom.
The inclusion of the online only requirement doesn't bother me as i have access to consistent broadband. It doesn't bother me that much. I'm upset because its obviously automatically alienating a segment of gamers who don't have consistent online access. But, the issue of single player is negligible. I played single player online in D2. I just made my own room with an unintelligible name and password, allowing me to play alone. It just gave me the option of playing online with my friends as well. Still ,though, as Max said, there's no reason it shouldn't be there, except that Blizzard wants to ensure players are exposed to their online systems, always giving them the temptation of dropping a buck or two on a nice shiny new item. Its their way of constant revenue from Diablo. They have constant revenue from WoW through subscriptions. They have constant revenue from SC2 through its professional competitive circuit. Now they want a constant revenue stream through D3 and they found it.
Of course, one of the tragedies of all this is that they re thus outlawing any form of game mods whatsoever. Its, in many ways, the advent of Console policy on the PC platform.