End Game
Monsters in Act 1 are level 61, in Act 2 - 62 and in Acts 3 and 4 - 63. And each of those have Tier items that ONLY they drop - so a total of 3 Tiers in Inferno.
Always online
Diablo 2 is a natively cooperative game but players didn't even realize there was an online component in it. There were also a lot of security problems - it was very easy to hack the servers.
Secret Cow Level and Easter Eggs
Actually, there is a REALLY big easter egg in the game, but I'm not going to tell you what it is...
Skill Interface
Players sometimes didn't know how to interact with all the systems. It's almost like they're saying 'If I could put the skills in categories I'd be able to understand them.'
The fun is in playing the game, not working out how to play it.
We make games that are very hardcore, but we still want them to be approachable, and it's one of those situations where you're never going to make both camps completely happy.
RMAH and the Item Economy
It's mathematical - you WILL find good items. It may not be the items you want, but you will find them. Then you can put them on the Auction House.
Originally there was a listing fee, and we struggled to find ways to remove it, and finally did - so that players don't have to invest money into the Auction House if they don't want to.
WASD and Mouse Controls
Characters only moving on eight axes - it works, but it doesn't feel awesome. Feels clumsy, not the Blizzard-quality.
The worst mistake you could ever make is to have two control schemes: you're basically taking the hardest thing in your game, and doubling it.
Hardcore
We have a prime directive of 'do no harm to the co-op game'.
We didn't want people to find ways to exploit, or to steal items from other people.
Having player corpses NOT be lootable makes it MORE hardcore, because what would happen in D2, is that you could play with your friends, and if you died you wouldn't lose your items, because they'd loot them for you and give them back.