The history. There are like 36,000 years of written history. That's pretty cool.
The magic. It's an odd mix of the medieval feel, with characters getting a chance to learn some cool spells and a real emphasis on the whole teacher/student thing.
It's big. There's something for everyone.
Because it's big, there are lots of characters...not only 1st lvl commoners, but also lvl 28 archmagi. That's cool. The world isn't static, and it's not waiting around for the PCs. Further, the PCs can't just choose to run roughshod over everyone else. Many other settings have population levels that just seem unrealistically low.
Just about everything in D&D can fit there...somewhere.
I like the amount of ruined cultures. It really gives the world a "lived in" feel.
I'm reading Swords of Eveningstar at the moment, and it really brings me back to the Grey Box feel of FR, which is I think one of the times I liked best.
The 3.0 FRCS was probably the best campaign set ting book I've seen.
Banshee