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.