Well, the Forgotten Realms has these traits:
-Detail. Its one of the oldest, still running settings. You won't lack for detail no matter where you look. This is good when you're looking for inspiration and a place to run a game. I think a good quarter of the more popular cities are fleshed out enough to simply explore. This is also terrible when you run into a Canon Setting Grognard and you're trying to run your own game in it. Ram up against some setting detail that is other than what you need, or miss some minor detail buried in a sourcebook and they jump down your neck over it. Annoying as hell.
-Long History. Seriously, the history of the Realms has more years in it than human recorded history and not too far short on detail either. See above regarding the issues.
-A history of imbalanced mechanics, all over the scale. Incantatrix(lets turn metamagic up to eleven!), Mythal Seeds, Sarruhk(open ended, this is going to end well), Harpers PrCs(nearly hopeless). This is endemic across 3.5 of course, but FR has a longer history than most, and thus more crazy stuff.
-Canon powerhouses. FR has a TON of epic or near epic NPCs still in active duty, and not much reason why they won't just swoop in and solve the situation in 5 minutes if they care about the stakes. Lots of DMs have pulled that too, which is why it irks many.
-The gods. They're...well, troublesome. AND they have a track record of avatarizing and interfering. See above.
-The Reset. WotC blew up the Realms for 4e. This did not endear them to anyone, at all.