I hate the Forgotten Realms because the worst and most uninspired D&D games I've ever run were set there.
We started playing with our own world back in the 80s when I was in middle school. This consisted of the names of maybe 10 kingdoms and areas sketched on a crude map. There might have been a few deities, but I don't remember. Not any history that I can recall and setting flavor was cribbed from B/X books. The focus was on the action of the adventures themselves, not on the world. It was a lame, vague backdrop and we had a lot of fun.
In 1988, I think, we got the FR Gray Box and loved it. I read and re-read it, absorbing the staggering amount of setting detail. Here was a whole world of adventure already created for us! Awesome! As a group we decided to pick up our ongoing campaign of 5th level characters and ret-con the whole thing as a FR campaign.
The change was significant. Suddenly there was a lot of background information--the names of NPCs and inns and towns and forests. Rather than having an adventure in some unnamed, untamed forest we had an adventure in the Spiderhaunt Woods. Everything had a name. Books and maps needed to be consulted. Careful consideration had to be given to the established lore in order to do it 'properly'.
The FR of the Gray Box has a neo-Tolkien flavor that was enticing in its specificity (coupled with the huge amount of material), but I soured on it quickly. Even for a 16-year old it seemed like imitation LotR--the trappings of and wistful preciousness Middle Earth without the rigorous mythology to support it.
Another mark against it: the ubiquitous presence of Ed Greenwood everywhere in the late 80s and early 90s providing additional material through the smug Mary Sue mouthpiece of Elminster. This is after the annoying amount of adulation given to his home campaign PCs and Elminster in the box set and that (aforementioned) awful Spellfire novel he wrote. They kept adding more and more pieces and I wasn't even liking the pieces I already had.
We played maybe 4 levels in FR and struggled through every one. I was too young and inexperienced to know what was wrong, but the games were so bad we jumped to other campaigns in other worlds--Underdark, Spelljammer, homebrew worlds that everyone found much more enjoyable. Eventually, in our primary campaign, we did more an more treks outside of the FR (other worlds) and then moved the whole thing back to my homebrewed world (which FR did inspire me to flesh out much more). We were far happier. Funny enough, I retired that world as having grown far too complex to run--the mythology runs about 200 pages!
I haven't run or played an FR game since 1991 or so and don't miss it a bit. It was the Drizzt novels that actually kept me in the setting for a few years beyond that. I think I read up through the Menzo(berranzan?) trilogy. I read a couple of others as well before realizing I could be spending my time reading good fantasy books.
Other observations:
*I associate FR with a good bit of the power creep in D&D. The FR source material is filled with 15-20th level NPCs. They're EVERYWHERE. "Bobar Glorpleks the Tailor, N, Bard 16" or some such. Our group's level 7 heroes, who had struggled through a ton of dungeons to earn those levels, felt outclassed. I'd always felt, coming from B/X, that ultra-high level characters would be renowned. You'd here tales of their deeds well before chancing to meet them. But in the FR everyone and their Uncle Urgglenup has 10+ levels. Do not like.
*I do not like the names. You have your choice among the Pseudo-Tolkienish, the cheesy fantasy composite names (Frostfell Mtns or whatever), and the random jumble of exotic syllables complete with superfluous letters. Of course, I have a soft spot for the Known World, which is worse with its awkward names and cutesy references (Yavdlom, anyone?).
*I always kind of liked the Red Wizards. If memory serves I might have used one as a Spelljammer villain, but that's about it.
*A friend had a sourcebook on the FR-version of Mongolia between FR-standard and Kara-Tur that looked interesting. I like the idea of a huge expanse of territory--thousands and thousands of miles...of adventure! That inspired one of my own settings.
So, to sum up, I couldn't make much use of FR as a campaign setting. I hated the games we had there. It felt too much like running something from someone else's laboriously created, and poorly conceived, game notes. I don't much care for many of the design choices that Greenwood and the others made in creating the setting and I don't like how it has taken over as the de facto D&D world. There are other flavors of D&D out there and I'd rather play those. To be sure, I don't play in any of the published settings anymore, but I've had the worst experiences with FR.
Out of the 7-10 people in our current group I probably have the most knowledge of FR. One of our longtime DMs has his own homebrew running for 25 years now and zero interest in the FR. Another DM has passing familiarity but has never and will never run a game there. Some of the others might have read a novel or played a game. That's about it. If the FR never existed, they would continue on playing D&D as they have without a bit of difference.
I myself won't run FR again. I'd play in a FR campaign without hesitation if it had a good DM, but I'll always prefer a homebrew world with lots of fun, unique and surprising details.