Back when there was a big hoopla going on about Fellaini to Chelsea some blogger did an in-depth analysis of him and claimed that while he is undoubtedly a great player, statistically, his abilities are not sufficiently in tune with those required of any particular role. Meaning that he loses too many balls and tackles at a level bellow that expected of a DM, he covers the field less than you'd want a B2B to, and he does not score, dribble or penetrate defenses at the rate expected of an AMC. So, while he has a lot of brilliant qualities, at a top level club where he'd be expected to deliver at a high level at the position he'd be played in, he would flounder because he wouldn't be good if he'd specialise at any one position. At a worse club where he'd be their best player and could play multiple roles, he would flourish.
I'm not saying I agree with this (I even blogged about how awesome Fellaini is), and even if true, all of the top sides barring perhaps City should have pounced on him, but there's something to it and maybe the top scouts have vetoed him for this reason. That said, I would love to see a club get him and stick him at CB, he'd be brilliant at it.