I feel very much the same way you do with regards to Samuel L Jackson, there is probably only really a handful of roles that Jackson has played where he really gets into the character.
However i'd say probably 95% of the time Jackson plays himself, from one film to the next, it's the same attitude, same coin praises *beep* etc... You're watching Samuel L Jackson..
I wouldn't say Samuel L Jackson is a bad actor, but he's not a great actor either, the major problem with Jackson is that he does too many movie's at a time.
No chance or time to really invest in developing the character he plays, unlike other actors who might not be doing as many films. However I think with Samuel L Jackson it's solely about getting as many films under his belt and money as possible, rather then the enjoying the art of acting.
Over recent years, I think his appeal and popularity has diminished overtime with fans, I have noticed a lot on message boards people stating they getting bored with the same characters he plays in his movie's.
I think though if he limited his number of film projects and focused more on the characters he takes... He would show more versatility in the personality of the roles he plays. I have seen it in some of his films, so I know he can do it.