It seemed to me that Michael Dorn's acting improved immensely during the course of TNG. I don't know if it's just due to Dorn growing more comfortable with the character, or the writers finding Worf's voice. Probably a bit of both, since they all had over 10 years developing Worf. I like to think that Dorn learned a lot working with Patrick Stewart, whom the cast reportedly nicknamed "Coach".
Anecdote: I saw Dorn at a small convention in Sacramento about 20 years ago. He said he used to stand at the tactical console, looking down at Stewart's bald head, and thought about throwing an egg at it.