uberdrops?

[Thecla]

My faith was restored earlier today though, when some nice people
got me past that boring act 3 jungle crawl and even helped
me kill Diablo. I figured I'd be levelling in nightmare diff
right now, but I obviously forgot to talk to someone in town
or something, because I didn't get the quest

Well, it could have been because you weren't close enough, but another reason you might not get the quest is because whoever created the game had already killed Diablo.

I manage to do this all the time. Just the other day, I was rushing an old lvl 10 sorc (my only char on a separate account that I haven't used in a long time -- curiously she survived Bliz's recent extermination of unused accounts, while a more recently used Matriach on another account went PNF, finally to be restored, after a couple of months of e-mailing Bliz, with a silly "_A" on the end of her name) so that I could activate the account for hardcore in the event that 1.10 ever arrives.

I joined some public games, and found someone to party with: after finishing off Andy in Act I, I brought in a high lvl character to get the staff and cube in Act II for my companion, and take out Duriel, then rejoined with my sorc to talk to TalRasha and get the quest. Then we went to Durance with my high lvl char and took down Meph to 1/4 life. I rejoined with my sorc and we finished off meph from a distance (my lvl 10 sorc using a lvl 1 charged bolt, and my lvl 15 barbarian compatriot using a bow; it took a while). He had to go, so I joined another game, with an amazon, who was slightly suspicious of my lvl 10 sorc in Act IV. I ran around the outer steps trying to kill off the odd doom-knight minion/boss with my lvl 1 static field and ice bolt (it would have been easier if I hadn't used up all my mana pots killing meph, so I had to keep portaling back to Jamela to refill my mana ball). Then I offered to clear out the CS with my high lvl zon if I could come back in with my sorc at the end to kill Diablo, which was agreed. So I cleared out the CS, killed off the seal bosses, and rejoined with my lvl 10 sorc to kill D. We opened the last seal. It was me, a lvl 26 zon and a lvl 20 barb (who had joined subsequently). They were having a bit of a hard time with it. I sneaked up behind D and static'd him down to about 1/5 life with a few deaths (I don't think the others really understood what a contribution I was making to the effort, in comparison with their puny attempts at damaging D); the worst part was that one flame ring almost -- but not quite -- killed me, so it was hard to sneak up without going near to death. Then they finished him off. That's when I realized the 'zon, who had started the public game, had already killed Diablo...

I even do it to myself. After starting a game to mule over some hc item, I figured I would go after normal/Baal with my hc necro, and get him his Destroyer title (he'd done the ancients). So I worked my way through the keep, and the throne, and Baal's minions, and Baal, only to realize that I had created the game with a character that had already killed Baal...

Anyway, the moral is if you're trying to complete a quest in someone else's game, always check that you can when you join the game.