Things I liked:
- The fact that Jude Law didn't play Dr. Watson like a bumbling idiot. Because guess what? He's not. He's also not a complete sycophant who follows Holmes around like a puppy. He's a respected doctor and a war hero and even a bit of a ladies' man. So thanks, Jude, for portraying Watson as he was always meant to be portrayed.
- That even though Guy Ritchie/Robert Downey, Jr. created more of an action-hero Holmes, there was still an aspect of deductive reasoning and analysis in almost every scene. Because that's what Sherlock Holmes does, and without this part of his personality, Sherlock Holmes just isn't Sherlock Holmes. He looks at a situation, takes in the most minute details, and comes up with a solution to the mystery. So I felt like it was an acceptable balance between physicality and intellect. Yes, you need some action (otherwise you're not going to keep teenage boys' attention, at least not in the year 2009), but Ritchie thankfully realized there's no need to dispense with what makes Holmes Holmes.
- The appearance of many of the characters from the stories (even if they weren't introduced in exactly the same way) -- Mary Morstan (Watson's first wife), Mrs. Hudson (the landlady at 221B Baker Street), Inspector Lestrade, Irene Adler (who only appears in one story -- although she's mentioned in several -- but has a relatively large part in the movie), and Professor Moriarty (although we never get to see his face).
- All the little details that you would only notice if you'd actually read Conan Doyle's stories -- like how Holmes is a master of disguise; how Watson has a minor gambling problem; how Holmes plays the violin and gets a bit, well, crazy when he doesn't have a case to work on (I noticed the filmmakers conveniently left out his cocaine addiction, though.); how Watson walks with a limp and uses a cane because he was injured while he was in the army; and the list goes on.
Fun Fact: There is a line in the movie that is taken directly from one of the short stories (and believe me, I was completely ecstatic when I heard it). Holmes says to Watson, "It makes a considerable difference to me, having someone with me on whom I can thoroughly rely." It's from "The Boscombe Valley Mystery."