For instance, when I was an impressionable lad I loved Sonic 3D Blast and Awesome Possum. Now I can't stand them. Sonic 3D Blast is boring and slow, Awesome Possum makes me want to go kill the furry twerp. I'm actually a bit embarassed I used to enjoy either of them.
I'm sure a lot of folks here can come up with a lot more interesting anecdotes about this subject.
Tony Hawk pro skater 2 on the GBC, it used to be so much fun, but now I find it annoying and boring. Plus it has the password system which has always been ass.
It just occurred to me I used to love FACEBALL 2000 for the original Gameboy. I thought it was amazing they could have a cheesy raycasted first person shooter on a handheld. Now all I can think is, "Um... no strafe, crap frame rate, a zillion levels and no save feature... why am I playing this, again?" The only good thing about that game nowadays is that the theme music is infectiously catchy.
I used to love the NES Mega Man games when I was a kid. Now I find them really frustrating withvery little pay off. The enemies and bosses have really cheap bullet patterns and the controls are really loose for how presice your jumps have to be.
Well, I played the heck out of Keith Courage on the TG-16, but that was only because it was the pack-in. I don't know that I even really liked it that much then, but I certainly don't like it much now.