Man, I could really write a whole book on D3 launch and patch history. It's some of the most mind-boggling set of decisions I've ever seen from a developer.
Yeah, my point about the lines of dialogue was precisely that, not that it's a negative to expand with voiced NPCs (I like that), but that it was executed so badly you didn't want to engage with any of it, so it ended up being wasted resources. I called them meaningless, because while I understand wanting to add some character to the mercs for example, they added nothing to the atmosphere or the world. There are only so many times you want to hear the scoundrel making jokes about his love conquests, The patron at the Inn that I mentioned, had some dumb lines like: "hehe I bet you are going to die in the church!", then when you came back, he would go "oh, you are so badass, I'm never betting against you again!". Just awful attempts at banter that killed the oppressing, gothic atmosphere you'd expect from Diablo.
The difficulty was a sticking point for the first two years. When the game first launched, the first 2 difficulties were nothing special (normal and hell), but the last difficulty (Inferno) was bad in ways it shouldn't have been. I would've appreciated the challenge and god I really tried to do it, but loot being so badly designed meant that in the end, the only way to get upgrades was through the auction house. You were only looking for 3 stats and nothing else. Oh, by the way, unique items were extremely rare in normal and hell, so most people would go through the whole story mode twice without seeing a single one. And when you found one, it didn't have the main stat you needed and most didn't have unique effects at all. 99% of them were worse than rares.
The main problem with it was the ilvl (item level) of the loot. Inferno difficulty started close to your character level 60 already, which was the cap at the time, but items had a hidden item level. Every item you saw in acts 1 and 2 were basically ilvl 55-60, and ilvl 63 items only dropped from Acts 3 and 4... The stat rolls for ilvl 63 items escalated so much in power (because they were the true endgame gear) that they made lower ilvl gear completely irrelevant.
It was the difference between a weapon having 120 STR and having 350 STR. So if you were a melee class in Act 2 getting close to the boss, but dying to fire damage you couldn't max your resist of, or don't have enough DPS to do it, what did you do?
You couldn't put more points in some abilities to power them up or specialize in a certain type of damage to counter monsters because the only stat that mattered was DPS. You couldn't farm the gear in previous sections because that ilvl gear you needed only dropped from the acts ahead of you. Then you go to the Auction House looking for a new sword, yours has 300 DPS and is level 60 already, pretty good right? You check and see lvl 60 swords with 800 DPS being sold by the boatload and you suddenly realize that bashing your head against the difficulty of the game was pointless.
The AH became the only viable way to itemize.
When talking about how balancing Inferno difficulty happened, the lead designer said something along the lines of: "we just made it as hard as we could until internally we could barely beat the game, then we just doubled that difficulty before release". The "we just doubled it" practically became a meme in the D3 community.
According to the designers, they meant for people to take months and months to clear inferno and only using the AH sporadically to finish up their character. That was their endgame proposition, just doing the story portion again in the hardest difficulty and chipping away at it for weeks. Nothing else. They did that because they hated that people in Diablo 2 did boss runs for loot over and over, so none of the bosses had standardized loot tables like they did in D2, instead they meant for you to do whole acts again and leave uniques to the luck of the draw.
Another thing they did was that Health Pools for Rare and Boss monsters were doubled for every member in your party, so teaming up in a multiplayer game became a negative. They actively discouraged it by doing that, as far as I'm concerned (I'm a solo player, and even I thought that was dumb).
Many of the patches in the first couple of months were also aggressively against the flow of the game and the needs of the players. Literally the first one they did was to nerf Attack Speed on weapons and jewelry. Just taking away the little power you had and conversely making entire weapon bases like swords irrelevant (because they were lower DPS weapons compared to something like clubs or Mighty weapons for Barb, but with the added benefit of attack speed, and like we said before DPS is all that mattered). Up until the day that Reaper of Souls released, there wasn't a single viable Barb build that could use a 2H sword because swords in general were irrelevant.
The following patch, they added enrage timers to Bosses and Rare monsters. People were struggling to clear some content with low ilvl gear, so to prevent players from taking on harder challenges and spending literally 20 minutes at a boss slowly chipping away their health so they could make some story progress, they added a mechanic that would make them practically invincible after a few minutes. Enemies would glow red, heal up and do increased damage when the timer was up. The idea was to tell players "don't engage with this content yet, you are not ready" according to the devs. Instead what most players did was get frustrated and spend time in the AH gearing up instead of killing things for drops. This was even worse for Multiplayer, because adding even one person to a party meant that your gear needed to be not twice as good, but four times as good to be able to kill bosses before they hit the enrage timer. DUMB. This killed Hardcore mode btw.
One thing people who still wanted to play multiplayer and get good gear did was to get full sets of magic find gear and go hunting for treasure goblins. People would party up, go to a different spot each where goblins were known to spawn and try to clear the trash enemies around them, then call their friends back, everyone put their magic find gear on (which was much weaker because it probably had fewer good stats on it), and then they would try to kill the goblin before it teleported away (they had so much health to that doing it solo with magic find gear was next to impossible). NGL, it resulted in some hilarious videos of Hardcore mode players dying after the goblin ran away and aggro'd monsters that were obviously to dangerous to kill in magic find gear. People did this for months btw. Literal months.
It says something about a game when people would go through all these lengths and basically not play the game (you know, kill monsters, and do progress), and instead they would sit for hours at a time bored as hell just using waypoints and checking random spots in the map for 1 particular type of monster, one that doesn't fight back, and put on a set of special gear and the end result was much more consistent and profitable than killing bosses for loot. At thing at some point it became about players trying to defeat the game's own bad design, like just outright refusing to play it the way it was designed. Diablo 3 developers just nerfed magic find after that.
God, there are so many more things they did, but overall, the first 2 years of the game were dull as fuck. Very, very few meaningful content patches you could count with the fingers of one had, and every time patch notes were up it was a nerf to something, whether it was movement speed, gold find, items that could drop from containers, etc. Just nerfing whatever alternate ways the community would find to make the game fun or porfitable. I did manage to make some good money on the AH tho, probably around $600, but none of this process involved playing the game, just flipping items. There were some good items every one could use, like Skorn, a high DPS two handed axe whose unique attribute was that it could inflict bleed (wow! so creative). Because one of its guaranteed rolls was a primary stat STR/DEX/INT it meant everyone could use it and had very few wasted stats. Other useful items were Natalya's set pieces for Demon Hunter because as a ranged class it was the best one to farm endgame items. Stormshields with high block chance were in good demand, The first few days each one sold for around $50.
tl;dr:
So how did they fix all of this? They just went the entire opposite way with the expansion. Made the game extremely easy, and made unique and set items incredibly overpowered with unique effects and set bonuses that no rare item could touch. Most D3 builds today are based around whole sets of unique or set items and Rares are relegated to 1 or 2 fill spots at most, when in Diablo 2 it's the complete opposite. Rifts and Bounties was a good addition, but compared to something like the Atlas system in Path of Exile it's nothing special.
The whole balance went away from them in a spectacular fashion. You start at level 1 with a 3 DPS weapon, collecting 1 gold at a time, 15 minutes later you have a 12k DPS weapon, dropping stacks of 5k gold. By endgame you have literally billions of gold to spend and abilities do 1200% weapon damage with 550k DPS.
Yeah, you are right, Diablo 3 did monster numbers, that 30 million figure was before the expansion iirc, just a beast, but it's a textbook example of how you can still fuck up so much goodwill with boneheaded decisions and poor design. A lot of games from that era, or even before that era still manage to pull numbers from design alone. Heck, Valve is a textbook example of a developer who couldn't care less about most of their games, and Left 4 Dead 2 probably still has better average concurrent numbers in the past 5 years than Diablo 3. You just have to not fuck it up.