This is a classic enemy-creating, I-used-to-respect-your-opinion kind of thread! My distinction is going to be that "overhyped" means they just get too much media attention and "overrated" means that they're more highly regarded by fans and critics than I think they deserve. And of course others will say the same things about bands I like. Onward.
U2 - It's not that I think they're talentless or lame. I just don't see the appeal. Lots of songs that sound kind of profound but never move me in any way. Many of their fans seem to think U2's music "changed my life" in some way. Baffling.
REM - You don't hear much about them now, but their fans are still out there. Back in the day, they were praised as a great band, but I never could see it. It was like some kind of bizarro world where bland vocals are called great, dull melodies are considered influential, and vapid lyrics inspire others. I saw them at their rock HOF induction concert (shudder) and was reminded again that I don't understand why anyone likes them.
Oasis - In addition to their dull music, they claimed to be great and not enough people reacted with laughter.
Radiohead - I've never understood what people see in this band. For me, the telling thing is that I can't remember their songs, even after hearing an entire album. They're the band that smug music lovers seem to claim to have found all by themselves. Every Radiohead fan I've ever met insists that I haven't really listened to them, and when I say I have, they throw up their hands and say I don't like music at all. Give me a break. Maybe it's their fans that annoy me more than the band. It's not like their music intrudes on my life very often.
Rage Against the Machine - Granted, they have some pretty cool guitar licks, but why do so many people insist that they're such a deep, socially-conscious band? Maybe I just heard too many garage bands in high school that had already said the same thing. It's not that I think their political activism is misguided. I just don't see how their message is that interesting. That, and I've never found De La Rocha's vocals compelling. Like if my auto mechanic suddenly thought he was a rapper.
The Beach Boys - This band is the epitome of undeserved praise from the 1960s. Their early stuff is a continuous rip-off of Chuck Berry. Their later stuff just fed off what every other band that was doing at the time. Apparently they were influential and many other artists are big fans, but I've never understood why.