Might get a bit of stick for this post, but I'll go for it anyway.
From my experience of watching United, it has now for a while been a team based around the quality of individuals or partnerships. Ferguson always liked to have at least one superstar in the squad, because he was aware more than anyone that the high end games are won by moments of superb individual quality.
It was for this reason that Van Persie was signed even though you didn't 'need him' as so many on here said at the time. You very much did need him. To an extent, you still do.
In recent years, it strikes me that you've moved away from a particular brand of football which I always saw as 'counter-attacking containment.' You let teams get to a certain point of the pitch, then pressed, won it back and scored.
This game-plan relied on top quality forward players, but it also gave you an identity and, in my view, was what created the fear factor.
What has been deteriorating, and seems to be something Moyes is trying to correct, is the pressing game. It's all a little bit lethargic in that area between half way and your penalty box. You're letting teams get a little closer to you, and you're giving yourselves further to go with it once you get the ball.
Where Moyes went wrong is that he thought Fellaini could be the destroyer to win the ball back. He's shown that he can't really do that well. Moyes thought he'd bought one half of the solution. The other half is a central creator; someone who is able to explosively start attacks from the half way line or deeper. Someone with the ability to spread the play or thread little through balls between a defence. That should be Kagawa but it really isn't.
So thus the issue with the football you're playing. Fellaini isn't playing the role for which he was bought, and you still haven't got the connector between the midfield and the striking quality. This means that the strikers are still scoring a lot of good goals, but there's a reliance on them pulling a rabbit out of the hat, as opposed to having a consistent threat throughout the team.