I take it that the OP is asking for stuff that is highly regarded in geekdom, but which we find overrated? OK, off the top of my head:
(edit: After re-reading what I wrote below, I noticed a trend: most of this is stuff that I find decent or even good, just not as great as the accolades would have it. A theme, I guess).
Yea, Firefly. It is pretty good but doesn't deserve its cult adoration, imo.
The Witcher. Again, OK. But lacks that special something that really good fantasy has.
Harry Potter. I read the first book and thought it was good for what it was, but I just find the whole Potterverse annoying.
Patrick Rothfuss. I really wanted to like Name of the Wind - started it three times, I think - but just found the tone annoying. It was just too...Rothfussian. He's a guy that if you watch him on any of the countless Youtube videos he's on clearly loves hearing himself talk and being the center of attention. I saw a fantasy author panel in which he must have spoken about 70-80% of the time, with four other authors fighting for the remainder. Robin Hobb and Steven Erikson were far more insightful and barely spoke.
Labyrinth and Legend. Two highly regarded cult darlings, both annoyingly 80sish. Labyrinth is decent, though, just overrated (imo! chill). Legend is just...nevermind.
"The MCU formula". I admit, I like a good superhero and most MCU films. And some of it is funny. But it also gets tedious, the "MCU formula." The humor can be great ("That guy's playing Galaga") or just tiresome (Guardians of the Galaxy - "Hey, aren't we just so funny?!"). I've got MCU fatigue and hope they do something different going forward, but doubt they will.
Grimdark/Nihilism in general. OK, I get it: Human suffering and violence is so kewl! Torture porn for the win! Isn't life unremittingly awful?! Frankly, gritty realism is overrated--especially when it is grittier than reality almost ever is. But I do really like GoT.
Anime. Not my cuppa.
Joss Whedon. Agreed what others have said.
Blade Runner. Its a good film, but kind of boring and hugely overrated.
Star Wars Sequels. I know, I know, don't get Zard started. But these are so utterly lacking in any of the great qualities the original film had, but just have better special effects. Its like creating an android of your high school crush - very nice to look at, but with no soul or personality.
Terminator films. Yes, even the first one. Well, the first two were pretty good, but even those ran out of gas on a pretty basic scifi idea.
Brandon Sanderson. Very good world-building and magic systems, but his characters are just flat and his books just don't draw me in. I don't dislike his books, but they're just luke-warm literary experiences that so many seem to think are great.
YA. Just...enough. OK, they're a valid customer base, but it just feels like turning over the same ground, again and again, just in new clothes and whatever the self-consciously social issue of the day happens to be.
D&D. Just kidding.
I'm sure there's more, but that's enough angst for now.
