I think when people talk about how a show was better in early seasons but got so much worse later, it's usually because they're just tired of the show.
I've gotten tired as hell of Monk, Psych and House, but I don't think it's from an actual quality drop, just fatigue. Doesn't help that USA always shows them in marathon form, ensuring that people get sick of them even faster.
And I really need to watch Sherlock.
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xfd dude, it's just a popularity contest on the internets.
-Vlado