I can point to an album here and there by a lot of band that didn't quite do it for me, but to me, that doesn't really fit what the thread is about. None of those are major disppointments. But I can point to a couple for sure:
Queensryche: In a nutshell, the series of events over the last decade or so that has left this once-great band a hollow shell of itself that feel like a laughing stock when compared to a time when people referred to them as "the thinking man's metal band" and actually meant it and could say it with a straight face. Operation Mindcrime was a metal concept album masterpiece. Empire, while decidedly having a very commercial edge, was incredibly solid and managed to garner mainstream exposure while maintaining the band's pencheant for complexity and layering of sonics and mood. Promised Land was perhaps their moodiest and most experimental. At this point in time, they were on top of the world in terms of musicality. But the band was imploding, which completely altered their writing style. It had begun prior to Promised Land, but the fact that the guys lived on the island in Big Log for the writing and recording forced them to work together. After that, for HITNF, while DeGarmo was still in the band, things were different and they did not write as they had in the past (which, to me, wasn't a problem because I love this album). Then DeGarmo left, and things were decidedly different. Unfortunately, it was not the right time for Gray to be in the band. Over time, the Tate family gained complete control over every aspect of the band and the rest of the band stopped caring and participating, and things have gone from bad to worse as the band's marketing has been one cash-grab publicity stunt after the next, the music is written by outside writers instead of the band, and Tate's voice has deterioriated as a result of him not caring enough to take care of it. As a fan, it has been painful watching this once-phenominal band decline to the point where their current output is something I wouldn't even notice or care about if not for their past legacy.
Def Leppard: I was a fan in the early '80s when they were considered a "rock" band as opposed to the pop band they eventually turned into. Although Hysteria was a huge move in the commercial direction, it still felt fresh and had an edge to it. The follow-ups have felt, in varying degrees, like a band who is just going through the motions. There have been some definite high points since then, but I feel a bit let down by what the band turned into vs. what they were. And given Joe's severe vocal deterioration, I have no interest in seeing them live again.
Journey: I lost interest toward the end of the Steve Perry years. Then they found Steve Augeri and things were good. This guy could sing. He could sound like Perry when he wanted to, but still had enough of his own personality and attitude that you couldn't really look at him as "just a Perry clone." I felt like he was a perfect fit. The songwriting also seemed to step up a notch or two again when he was in the band from where it was on the last two Perry albums. The band had energy and were, in a nutshell, interesting. Then he left, and...as much as I like JSS, he was doomed to fail in that role. And Arnel, while a talented singer, does nothing for me whatsoever. Journey now seem stale and uninteresting and seem supremely content to be a nostalgia act. And that's fine. But seeing them with Augeri, where I felt a genuine excitedment and enthusiasm (which, I must say, is really an anomaly for a band that has been around that long and has broken up and reformed years later with a brand new vocalist) that has since left again.
Those are mine. I could probably add Metallica as well. While they've also had some high points since the Black album, it just feels like they lost their roots after that and have no idea what kind of band they are supposed to be. DM recaptured a lot of their core sound, but still somehow felt forced to me instead of sounding fresh.