Okay, so, I was going to start off by linking to an old post from John K.'s blog (the creator of Ren and Stimpy, the Ripping Friends, George Liquor, and renowned misogynist/bitter has-been), but not only can I not find the post, looking through all his remaining posts about Bugs Bunny it became evident I wasn't going to find anything nearly as relevant to the topic I wanted to touch on. It was basically this- in old cartoons (in this case, the Bugs Bunny short Hair-Raising Hare, directed by Chuck Jones) there is often a head animator and then various other animators working off storyboards, doing in-betweens and generally going off model. While the overall feel is concrete and even, when looked at carefully, you can clearly see the little differences and model changes in Bugs from sequence to sequence-- from controlled and well animated, to loose and slightly more amateurish.
This effect can still be seen today in contemporary children's cartoons, most notably (to me) The Marvelous Misadventure of Flapjack, and Adventure Time. The extremely bold outlines and simplistic, clean art allows the most minute changes to design to show, and while this is often done on purpose for comedic effect, sometimes it's just evidence of different artists and animators letting their unique styles show through. This sort of thing was frowned upon for some time, as producers and such considered these little differences to be blemishes on what they perceived to require absolute perfection and attention to detail-- but in effect that mindset may contribute to why flash-made cartoons, which tend to stay on model, come off as cold and sterile.
In the Flapjack episode, "Gone Wishin'", there was especially little done to stay on model from sequence to sequence-- while it was much more noticeable when done this extremely, I still found it promising and charming to see individual animators and artists' styles crop up, rather than the same model Flapjack and K'nuckles in every single episode. I took some screenshots from the episode to show.