My tastes have been somewhat variable through the years, but not a huge amount. There are a few things that I used to love that I don’t understand any longer (I was completely addicted to “Trapper John, MD” as a kid. WTF?)
What books, movies, shows, music have you gone from “OMIGOD, this is the best thing EVER!” to “I would be happier never to see that again”?
My girlfriends and I watched “Flashdance” almost every weekend in high school - I watched it a couple of years ago, and while I can see the allure for teenage girls (and boys, frankly), it was…not a great movie.
My ex-husband. 
As a teen, I thought the song “Through the Barricades” by Spandau Ballet was the DEEPEST THING IN THE WHOLE WORLD.
Cottage cheese
Pickle and pimento loaf
bananas
dirt (yeah, I remember eating dirt and liking it when I was a kid)
Fig Newtons
(Just realized OP uses the word ‘tastes’ but uses no food examples, if this is OT, sorry)
Any of the shows from my childhood. Gilligan’s Island–watched that the other day with my kid and she was like WTF!! I had to sort of agree with her–what an awful show, but I used to love it growing up.
Another one was Starksy and Hutch. I LOVED this show as a teen–terrible acting and just a bad show overall now looking back.
OK, music example;
Yvonne Fair’s album The Bitch is Black (I had nothing to do with the name) has a song on it called “It Should Have Been Me”. I remember loving that song way back when, I finally found a CD of the album on Ebay, ordered it, and when I played it, I was like, WTF?!?!?!.
Have no idea what happened, it is the same song, even the same ‘take’ I recall, but it is like finger nails on chalk board now.
If we count things we liked as kids, there would be a ton. My brother and I used to watch Brady Bunch every day and I adored the old Dennis the Menace TV show.
More recent, though:
Austin Powers - I know, a lot of people still like it. I saw it recently and except a few brief moments, I found the whole thing terribly unfunny now.
Saviour Machine - Used to be my favorite band. Now, I see how lame they were in many way. Just not my cup of tea, anymore. I guess they were OK, but I used to insist they were the best band ever. I was clearly wrong. 
[Katy Perry]
“I ate some dirt and I liked it…”
[/Katy Perry]
As a kid I used to love The Brady Bunch, now I watch reruns and can hardly get through them. The other day Alice was trying to catch a mouse or some shit and it was like “Who the fuck cares?”
I still really love The Brady Bunch Movie though.
I chew ice cubes now. I have been told this condition is called ‘pica’.
When I was a younger nerd, Monty Python’s Flying Circus would make me laugh until my sides hurt. Now I can generally take it or leave it, although there are certainly a few classic sketches that still make me smile.
Most of the cartoons I watched as a kid.
I can still watch Transformers, laughing at the stupidity of parts of it. (That I’m still a fan of the franchise in general probably helps.) Disney stuff stands up. Robotech hasn’t flagged. Surprisingly, the Littles actually stands up pretty good.
But GI Joe? He-Man and She-Ra? Voltron? Alvin and the Chipmunks? Jem past the first season (first season’s actually pretty good, though)? Good goddess, no. I actually actively refuse to rewatch a lot of stuff, now, just to avoid sullying it like I did.
About the only thing that disappointed me was rewatching My Favorite Martian. I liked it as a kid but the last time I saw it, it was awful (to be fair, the show jumped the shark after a season or so and I stopped watching; this may have been after I had stopped).
Other than that, I haven’t seen anything that brought that reaction. But remember, my tastes as a kid were not usual. For instance, when I was twelve, I decided the best movie I ever saw was Dr. Strangelove. I’d watch things like Gilligan’s Island, but I new they were bad even then.
Me: duster
My wife’s answer: leggings
Nickleback used to be my favorite band EVAR.
I’ve been a martial arts film fan since I was around 5 years old. A channel here used to show old Run Run Shaw and Golden Harvest films, and then I watched the newer hollywood stuff like Jean Claude Van Damme, Karate Kid, Chuck Norris, Steven Segal, etc. Then, my girlfriend took me to see Rumble in the Bronx at a foreign film theater, and I realized how bad Hollywood films were in terms of choreography, speed, cinematography, etc. Recently it’s been better though thanks to using Hong Kong choreographers, cameramen and fight coordinators, but when I watch the older stuff nowadays, I wonder why everybody is moving so slowly, like a bad musical.
I Love Lucy and Weekend at Bernie’s. Both were side-splittingly funny to me and now I can’t watch either.
Gino Vanelli (WTF?)
Gary Wright
The Man From U.N.C.L.E.
Hazel
Bachelor Father
Anne Rice. Oooooh I was so edgy.
Evanescence. Oooooh I was so edgy.
Super baggy jeans. Oooooh I was so edgy.
Sailor Moon. Oooooh I was so…nevermind.
**Two from the '60s:
Bonanza** - must see TV in 1965. Completely unwatchable today.
Bewitched - Darrin was an asshole. Endora was absolutely right.