Do me a favor, replace Forgotten Realms w/ Star Wars and replace Elminster w/ Luke, Dove w/ Han, and so forth.
Now, come w/ me on a little thought experiment ...
GM: hey guys, I'd like to run a Star Wars campaign. You know, fight the Empire, fly around in spaceships and stuff.
Player: cool! I want to be a smuggler and fly around in a fast, agile spaceship like Han does. Oh, and I want it to be kind of a shock b/c the thing looks like a hunk of junk, but then blam! afterburners and stuff.
GM: yeah, sure, you can do that. But, I want to let you know, your ship will never be half as cool as the Millenium Falcon.
Player: well, how do I get a ship as cool as the Falcon?
GM: sorry, there's no way to do so.
You get the idea ...
That strikes me as spectacularly unfun. Also, you guys can say we can edit out the various NPCs in the Realms. As I've mentioned that's a very viable way to approach things. But, it is a pretty serious revision of the setting. If memory serves, like every nation, settlement, pile of shacks I can think of has some 20th level uberdude associated w/ it. So, you need to come up w/ plots that continually remove them from play. Suppose someone is laying siege to Cormyr. Well, then doesn't the uberwhatever associated w/ that nation do anything? It requires some serious work, and will probably eventually strain credulity. And, I think the setting makes this harder b/c, and someone correct me if I'm wrong, there are a lot more good uberNPCs than evil ones. And, a huge amount of the printed material involves these uberNPCs in one way or another.
Listen, no one is contending you can't play the Realms. And, I think everyone has acknowledged that there are nice things there. But, the title of this thread was what we thought was wrong w/ them. So far, the best answer has been "you can ignore a large portion of the printed material." Sure, well ... that shows you there's a viable criticism to be made, right? It's not impossible to work around, but it does require a workaround.
I do prefer my worlds to be a bit more open than that. Or, more to the point, I'd rather have the world stacked w/ villains of vast power (viz. Diablo, though it's worth noting that you don't
play the worthless Marius, you play someone on par w/ Diablo himself eventually) rather than heroes, or failing that douchebags (Elminster) of great power. And, it occurs to me now that a lot of those NPCs are probably just put in there to stop the PCs from exerting any influence or changing the world, which is disappointing.