Never played pen and paper RPGs and I only learn the systems to the degree you have to in order to play. So, for me it's basically a setting, i.e. The Forgotten Realms is like a Tamriel or whatever for me.

Most people misconstrue the Lawful Good alignment. With any alignment, there's some wiggle room. With Lawful Good, it's no different. Truly Lawful Good was one of the more fun alignments to play with a DM that understood alignments are more like guidelines than rule sets. Maybe it's my bias, but playing a Lawful Neutral bounty hunter that later classed into a Lawful good paladin (with neutral tendencies) was quite a fun character arc to experience.
You knew this was coming.

To me it's firstmost the memories of playing first edition AD&D with friends when I was young. A blend of memories composed of the ruleset that makes AD&D, travelling to Amsterdam where the two elder players were living (one full time Dungeon Master, one player in one campaign and DM in the other). The rest of us were still in secondary school. Furthermore the characters we played of course (it's a roleplaying game for a reason and in various degrees of seriousness we took roleplaying serious), the soda drinks and chips that went with it (we never combined AD&D with alcohol, not even at a reunion we had when we were well in our thirties or forties). And a memorable instance of 'doorhalen' (I don't know the English word): playing on all through the night, with pizza for supper and breakfast in the morning after a whole night without sleep and full of Dungeons and Dragons.

The second thing AD&D means to me is CRPG in the Forgotten Realms setting, with the 2nd Edition ruleset and the Infinity Engine, playing the Baldur's Gate games and Icewind Dale.

The whole spin-off, like the cartoon series, the comics, the movies (all varying degrees of funny but bad) and the books by Salvatore that I've read on Elminster and Drizzt (mostly good) comes only third place for me.

I never played table-top RPG's. So for me DnD is pretty much the setting that such games as Eye of Beholder and Baldur's Gate based on.

That game my mom wouldn't let me play when I was a kid. :'(

I respect it for what it is, and how it helped inspire RPGs as a whole, but there are better tabletop games derived from it, such as Pathfinder and so on

a crispy bit of cheese. with some spice on it? kind of tex mex?

I think the English term for this is "all-nighter." Which would be a noun.

"We pulled an all-nighter playing Spades in my uncle's basement. We didn't go to sleep until 8 in the morning!"

That's me, for the most part. To me it means "generic fantasy that most probably has many elements I've seen before in P&P or videogames I've played", but not much else. I like some of the settings previously connected with D&D (Planescape, Dark Sun, Ravenloft, Kara-Tur), but I've never cared much for the mysteriously popular Forgotten Realms.

I may be doing the setting a grave injustice, but I can't help but think that Forgotten Realms got popular because Drizzt, and Drizzt got popular because angsty teenanagers :P But like I said, I don't really know the setting well enough to pass judgement. When I played D&D we either played Greyhawk or some homebrew settings. And even in Greyhawk we didn't exactly immerse ourselves in lore :P

I'm suspecting the reason why it's the most popular one is the very same reason why I don't care much for it, namely that it's the most generic high fantasy setting that immediately feels familiar to everyone right from the start. And I just prefer to be surprised and explore more unusual settings that also feel more coherent than "here's a bunch of ragtag fantasy clichés for everyone coexisting next to one another". Yes, I'm biased, too. ;)

GrayHawk would have to be the most generic. Forgotten Realms likely got popular due to the excellent novels from Salvatore (the aforementioned Sojourner series starring Drizzt Do'urden) and several others written in the setting by a slew of authors. The Red Wizards of Thay is a pretty good book, by the by. Dragonlance was also pretty popular due to the excellent books by Weiss and Hickman. But they never made a large transition to computer gaming for reasons I can't fathom, unlike the Forgotten Realms. That one is the setting for most PC D&D games including BG, NWN, ID, Demon Stone, EotB, Menzoberranzan, the Pools Gold Box series, the newer Pool of Radiance, Hillsfar, and I'm sure others I have missed.

I have to admit I know next to nothing about Greyhawk. I always thought it was kind of low fantasy in a Conan style world, or did I confuse it with something else? Does it have elves and dwarves and halflings and orcs and all that, too?

Dragonstrike was still quite unique and interesting example though.