I'm a liker. For me, it's the depth of the world - an incredibly rich history that gives adventures in the realms a context that most other settings can't really match (reading the Grand History of the Realms can give you an idea about just how complex the setting is). The Star Wars and Warhammer universes are the only settings that I can think of that have that kind of depth.
I also like its kitchen sink factor, or liked it, before they decided to blow up every country that wasn't psuedo western European. If I wanted an Egyptian, Greek, Babylonian, Persian, Spanish, Aztec, or just about any other adventure theme, there was a place for it in the Realms. Sure, the analogues could tend to be a little too close to the real world, but mostly they worked well and there were plenty of nations that were pure fantasy constructions built into the realms as well. (As an aside, I tend to disagree with people who don't like real-world analogues: I find them an excellent way to instantly evoke a culture, architecture and theme without having to subject players to extended exposition. Cultures without a historical or literary antecedent are very hard to pull off in an RPG. There are a very few settings I can think of that successfully managed to build a "culture" from scratch -- Planescape and Dark Sun come to mind).
Without question, though, the Realms could use a major cleanup. The number of realms shaking events and the proliferation of super-heroes in 2E and 3E made the setting a total mess, while the 4E solution (blow everything up, advance the timeline 100 years) was just awful. I hope that 5E, if and when WotC get around to it, papers over those events and gives us a "clean" setting to work with, one where there aren't a million overpowered good-guy NPCs running around and can serve as a starting point for an epic player campaign that won't be run roughshod over by metaplot and novel characters.