I don't what it is about Robert Downey Jr., but he has an uncanny way of making subtle, witty humor absolutely hilarious. He and Jude Law were great together, and the not-so-subtle play on the perceived homoerotic relationship between Holmes and Watson made for a very funny duo.
Aside from that, the plot was well-constructed, the dialogue was well-written, the action scenes were supreme, the cinematography made for a unique and refreshing atmosphere, and the "mystery" was pulled off very well.
If I had to take issue with anything, it'd probably be Rachel McAdams's acting. It seemed to me like she was there for s*x appeal and as a set up for Holmes' witty dialogue.
I don't think anyone who went to this movie went there to actually see a great film. I think most people just expected a cool 3D thriller that you don't get to see very often, and that's pretty much what it was. In terms of acting, directing, writing, and originality, this thing was pretty terrible...but, with the 3D gimmicks, the unnecessarily up front nudity, and the brutal killings, it was definitely entertaining and fun to watch. The 3D parts of it were pretty awesome, but they fell short of expectations, and the movie was sometimes blurry with the glasses on.
In short: go see it in theatres if you're curious about the 3D aspect or you're easy to please.