As WoW attempts to solve its issue of horrible pugs by making the tanking role as brain dead as the dps role Diablo 3 is poised to take over as the default method of having a fun group experience in an online game.
In Diablo games everyone is dps. If you want to pull just shoot something, no need to wait for someone else. If you're taking too much damage just run back behind your team for a few seconds. Mobs in Diablo games have an in-built hesitation mechanic so if you move they hesitate very slightly, allowing you to reduce the damage you take. Friends can help by ping-ponging aggro, by overthreating your threat value so monsters stop chasing you. It's a more agile more immersive playstyle.
Diablo 3 will take a lot from what Blizzard has learned over the years in WoW. Instead of the freeform games of Diablo 2 (for example Baalrun95) D3 players will set up games by choosing where they want to quest and how many people they want to quest with and the system will build a party for you to suit that. Which is quite similar to how LFD works in WoW, just without the queues.
Does it lose tactical depth? Yes, but WoW is withdrawing from the style of tactical depth as fast as it can. Who would want to play a game for the tactical depth of trinity gamestyle where it's impossible to rip aggro off the tank? That's not tactical depth.
In other ways D3 is simply streets ahead of WoW. Let's look at loot.
- in WoW you will see nice things drop and someone else will get it. In D3 if you see it, it's yours.
- Diablo games drop more loot that WoW-type games. Monsters are individually much weaker, you can kill a swarm in the time a WoW player would kill one mob.
- some WoW loot drops as currency which you then need to save up until you gradually save enough to buy an item. It's loot by installments. In D3 if it drops it's an item and you can use it immediately if you like.
- WoW loot is usually Bind on Pickup. In Diablo games if it drops any of your characters can use it. In D2 I quite often had the experience of finding an awesome item and thinking, "Wow, I've got to go make a Sword Barb now!"
- in D3 if you don't want it you can cash it in for real money or game money or crafting materials.