"Completely harmless" is not an accurate description of TV Tropes Wiki - many people have noted that it can become an immense time sink. In my opinion, this is due to the large amount of material being available and heavily crosslinked - and given high average and moderate variation in data attractiveness, this produces a variable-ratio schedule of positive reinforcement, leading to addictive behavior much like that observed in gambling.

That last link is the only safe one, by the way - it goes to psychology.about.com, as opposed to TV Tropes or xkcd, and is very brief.