In my opinion,
Arthur C. Clarke's
2001: A Space Odyssey series is perhaps the classic case of
wharfinger's law. The original is brilliant,
2010 is quite good,
2061 is tolerable, and
3001: The Final Odyssey is incredibly boring.
I blame this on Clarke's tendency to replace intriguing plot with long-winded descriptions of mostly boring futuristic technology as the series progressed. Character development also seemed to disappear into thin air near the end of the series.