Led Zeppelin - II -> III, Physical Graffiti -> Presence
Anathema - Weather Systems -> Distant Satellites (not a terrible album but not great either)
Iron Maiden - Seventh Son -> No Prayer
Dream Theater - Systematic Chaos -> Black Clouds...
Opeth - Watershed -> Heritage
King Crimson - Lizard -> Islands
Megadeth - Peace Sells... -> So Far, So Good, So What, The System Has Failed -> United Abominations
Muse - Black Holes And Revelations -> The Resistance (not a terrible album, but very, very inconsistent)
Nightwish - Once -> Dark Passion Play
Pink Floyd - The Final Cut -> A Momentary Lapse of Reason
Riverside - ADHD -> SoNGS
Rush - Power Windows -> Hold Your Fire, Counterparts -> Test For Echo
Not many of these are Awesome -> Terrible, but more just sudden, precipitous drops in quality.