I may have mentioned a few times on a few boards that I'm a huge fan of the Forgotten Realms 1e gray box. I got it when I was 13 years old. Up until that time, I used the Known World as presented in Isle of Dread and Red Arrow, Black Shield.
What set the Gray Box apart from everything else:
1. Great maps. Sure, compared to later versions, they were largely blank and kinda plain. But all those blank spaces just screamed wilderness adventure to me. And with our Wild West style game, those open spaces were often just plains with huge herds of free-roaming bison and savage orc tribes and the occasional wandering elf-with-no-name.
2. Mercenary companies. I don't know why, but the inclusion of the sample mercenary companies was about the coolest thing ever when I was a kid.
3. The Zhentarim. A group of evil mages. Who were merchants. And who didn't mind killing people who got in their way. Manshoon is still the lead dog in my campaign. Fzool is just his toady.
4. Waterdeep. Not a ton of info in the box, but it quickly became my favorite rpg city once FR1 was published. Even though I used it as a far-off city that the occasional worldy, wealthy traveller came from for my friends to meet.
5. No god-like NPCs. They come later. At this point, Elminster is just a crabby old sage, not a super-powered world savior.
6. Iriaebor. My second-favorite city. The blueprint for Sharn. Luckily, never over-developed.
7. Evereska. A mountain citadel full of elves. Best idea ever.
8. The NPCs. Lots of good stuff here. Including stuff like low-level thieves the characters might meet. Heck, at this time, even Elminster was cool.
9. A detailed history. I'm a sucker for these. The recent battle in Shadowdale was really cool, and it's affects on everyone was well done.