I think REM lost it at around the time their lyrics stopped being unintelligible. Once you could hear what Michael Stipe was saying, it was possible to notice that a lot of the lyrics were rather dumb, whereas before it was possible to insert one's own thoughts into the parts that couldn't be understood, thus making it seem a lot more personal. But musically also, they became increasingly boring and generic. The last halfway decent album was that one with Kate Pearson on it, and even that is a far far cry from the brilliance of Chronic Town, Murmur, Reckoning, and Fables.