Positive points:
Good defensively. Wins headers, defends corners and set pieces well and is difficult to shake off the ball while holding it. Is used like a wall to add rigidity.
Negative points.
Offers nothing on the ball though. Very very poor with the ball at his feet. Struggles to make to make basic through balls. Struggles whenever asked to move the ball and then make a pass.
Actually, struggles would be an understatement. He just can't do these things. If we see him with the ball then we need to stop expecting him to do anything other than pass it to the nearest player, even if that player is a CB.
Could see exactly what he offers yesterday, and believe it or not, teams do struggle to deal with players like him. He is very difficult to shake off, and, in offensive areas, can set up his teammates nicely. Also, he is very difficult to deal with while defending, not because of his footy skills, but because of his simple brute strength. He is very strong.
His contribution yesterday was purely defensive. Offered nothing in the attacking front. Mourinho, however likes to use defence as a means of attack and so Fellaini still fits in his plans. I even think that had he been there in the Swansea match, we would have managed a 1-0 victory. He would have bored the life out of everyone and driven fans crazy, but we would have come out as winners. It is almost like a paradox. A players with almost negligible football skills managing to somehow help us win games.
Everyone would love a better ball playing player in his place, and we will definitely get that next season, but for these final few matches, we need to tolerate and support him.