It definitely wasn't the acting, as I already mentioned Downey Jr. and Law are terrific together and every scene they share together are some of the best parts of the movie. It seems as if Downey is attempting to create a sort of Johnny Depp/ Jack Sparrow character with Holmes, and to my surprise he managed to do so very well. It isn't as great as Jack Sparrow but it sure is enjoyable.
So if it's not the acting that hurts Sherlock Holmes then could it be the writing? Well it's definitely a part of it. The dialogue between characters is great, but when Holmes is describing how he solved his mysteries he often gets way to scientific for the audience to keep interested.
Is it the plot? Maybe partly, as it turns out it's more interesting when Holmes and Watson have there bromance drama then when they're trying to solve a mystery. The demonic thing is kinda interesting and is a lot darker then you would expect from a christmas blockbuster, but the evil characters are not fully developed and some of the bad characters cause more laughter then fear. Most of the action scenes are pretty boring and nobody ever seems to have a gun when they need one, plus shouldn't sherlock have figured most of it out once they found the Ginger Midget?
I think the main thing that ruins this film is the direction by Guy Ritchie who for some reason thinks it's necessary to explain what's about to happen right before it does as well as go randomly into slow motion so you can watch run back and forwards into bombs (it looks even worse then it sounds). Ritchie nearly ruins this film by over directing it on several occasions. He creates a nice atmosphere but fails to bring anything else to the film, leaving Downey Jr. and Law to clean up his mess for him.
And on a parting note. I have to ask did anyone know who Moriarty was?