It seems like the biggest problems that great groups from the past have when they put out new albums years later is the songwriting. For whatever reason, songwriters seem to do their best work when they're in their 20s and 30s, as a rule. When the bands are older, the playing is usually still good (although the vocal ranges aren't always what they were "back in the day," as Robert Plant would surely attest), and they're often still pretty good live performers, but the new songs are frequently a lot weaker. The Rolling Stones are a prime example of this phenomenon, I think.
