Cahill is tenacious, hardworking, mobile, capable of mixing it and has proven he can tackle over the last few games. The only thing he's lacked (as a midfielder) is the right partner and a bit of disipline.
Fellaini is laid back, languid, isn't a workhorse and can't tackle to save his life. I honestly don't think I've ever seen him attempt a slide tackle.
What Fellaini is good at is holding the ball up, winning headers and linking things together with intelligent touches and bits of play. I think a bit of realism is needed to be honest, that is his position and it suits him.
People trying to make him into something he isn't, just let him play to his strengths and develop in the role that he is obviously suited too. Not Moyes is going to work a miracle and turn him into a Vieira when it's quite clear they are completely different.
Peace and Love.