I'd be happy to elaborate. Basically, you want the characters to speak like they are from that setting, so you use the "frontier" dialect. Here is an example: "Man's got a right t'express an opinion." This sort of thing is great as is gives the reader insight into the character and the setting all by showing and not telling.
Now, in exposition, you want to avoid language like that. Let me give you an example: "...but Bill didn't have a taste for fighting tonight." This line is used, not as something that the character said, but as something that the narrator has said. In essence, it is something that you as the author have said. I doubt you speak like this in normal life. Furthermore, it limits your ability to employ the really creative (and some might say pretentious) metaphors that make great writing a joy to read.
Basically, you want the drawl in your dialogue, but not in your description. Or, to put it another way, characterful language is a lot like whiskey. If its sippin whiskey your after, you don't want too much. If you just don't care, then you get rotgut.