While it seems to be a trend here, System of a Down is probably the band I've experienced the sharpest decline in interest for. I used to listen to them quite a lot when I was first getting into metal, but now I can't stand their angsty melodies, spastic dynamics, weird vocals, and, of course, annoying liberal rants. There are few things more irritating than politics in music.
The runner-up band is Rammstein. They were pretty much the band that got me into metal, and for a long time they were my favorite, but I rarely ever listen to them these days. I still find their music enjoyable, but I'm just not into the industrial sound anymore, and of course, if you read a translation of their lyrics, most of them sound pretty adolescent and silly.
Other honorable mentions:
Cradle of Filth
Type O Negative
Nightwish
DevilDriver
X Japan
Children of Bodom
Sonata Arctica
Stratovarius
I'd say I still like all of the above bands (besides SOAD); it's mainly the style of music that keeps me from listening to them. Melodic death, goth, power, etc. all sounded really cool when I first heard it, but got old pretty quickly. Still, I wouldn't be surprised if I got into them again sometime in the future.