The Autumn Offering - Their first two albums had some amazing guitar work, great riffs, and thankfully only one whiny part in one song. Unfortunately, after their second album, their ENTIRE LINEUP changed, whiny vocals everywhere, and it pretty much devolved into a generic metalcore.
Metallica - I don't want to explain this. I really don't. They began as one of the most influential thrash bands ever, and just got worse from there. To this day, I still consider St. Anger to be one of the worst albums ever made.
Ozzy - I've never really liked everything from one of his albums, but he definitely has gotten worse over the years.
Black Sabbath - Ozzy got fired. I don't like much of post-Ozzy black sabbath at all.
Static-X - Their first album was one of the best industrial albums ever. Afterwards, they kept on trying different sounds and experimenting, and their last few albums sucked so hard it's unreal. They've only made 2 songs that I like in the past 4 albums.
Megadeth - Their best days are certainly behind them, and they are still going at it. Don't get me wrong, they're still pretty good (to me at least), just not as good as they were before. I will say that modern Megadeth is leaps and bounds better than modern Metallica, thats for sure.
Rainbow - Things have never really been the same for me since Dio got fired. At least they made a few instrumentals after they changed singers, unlike SOME bands. >.>
Bethlehem - Also never have been the same without Rainer Landfermann. Best shrieks and screaming vocalist ever, and they lost him. Tsk tsk tsk.
Guns N' Roses - Slash left.
System of a Down - To this day, SOAD is my favorite band and I love everything they ever made, I still feel as if Mezmerize/Hypnotize wasn't as good as their first 3. The creativity and sound was very unique, but I couldn't help but long for Serj to sing most of the time. I will give Daron kudos for Lonely Day. However, if SOAD had a version of Serj singing it, and I heard that one first, I might think about it differently.
Extreme - Their first album = incredible. And from there.... I'm left unimpressed. They actually reunited and made a new album after Guitar Hero: Rocks the 80's when 'Play With Me' was put in. The album itself was kind of mediocre in my opinion. Probably one of the few bands who are dragged down by not being heavy.
Dragonforce - *sighs* Over time, it seems like they're losing their creativity and becoming less melodic and focusing more on effects. Sad thing is, they'll wallow in their success for Through The fire and flames no matter how absurdly overplayed it gets, and still remain popular. Also; Worst. GODDAM. FUCKING. DRUMMING. EVER. ARRRRRRRRGH!!!.