Some of Madsen's career choices have made me seriously want to hate him, but only because of all the wasted potential. Seriously, though, I think so many QT fans like him just for the simple reason that he was born to be a consistent player in QT movies.
What I'm trying to say is, in Tarantino movies, there are actors that do a great or decent job in his films as they probably would in any other movie and then move on to other things some which are just good, even better, or far worse than Tarantino's stuff:
Robert De Niro
And then you have people that were just born to be Tarantino players. They may be good actors on the whole or they may just be bums who can't do anything without QT, who knows? But, in any case, these guys and gals click with QT's work like no other set of actors. They bring his work to a new level, and in turn, he brings there work to a new level, and things aren't ever the same with the two unless he or she as the actor and QT as the director are working together:
Samuel L. Jackson
Madsen, like Thurman and Jackson was just meant to be a Tarantino regular. He utters the kind of words like Tarantino would utter them...if he were cooler, or blacker, or a woman, or what have you.
Anyway, thats just my theory. It still has some kinks in it, and it may be entirely wrong, but I'm standing by it for the time being.