I'll take a stab at this one. because it was a simple fact that rudy was limited from a skills standpoint offensively. it wasn't an issue of effort. it wasn't an issue of improvement - as limited as he was he got better offensively throughout his career.
in the case of dm, it's an issue of effort. he has the physical attributes, athleticism, etc to be a very good guard defender. but he doesn't give a ****. he doesn't try and on top of that he's gotten gradually worse as his career has progressed. fans have a hard time with players who don't give effort defensively and who don't work hard to improve.
yet, all that said, i don't hate mitchell either - i just think it would benefit the jazz to trade him (especially following the rudy trade) for another great haul of players/picks if they can get something done.