United play in a manner which is counter to what most call "modern football". The popular way of playing is a take on the Barcelona style, play lots of one and two touch football, short accurate passes, work the ball centrally and try and slot passes between the centre halves/full backs for forwards to run on to. It's how City play, it's how Chelsea play, it's how Arsenal play, it's how Swansea play and it's how Liverpool play. Some sides vary from this, but normally because they can't afford to bring in the style of player needed, think Stoke or West Ham. United do not play like this. United play like it's still 1995. They play with natural wingers (Young, Valencia, Nani), they get the ball out wide and they deliver crosses into the box. I'd be willing to bet that, if we were to look at which side has attempted the most crosses this season, United would be head and shoulders above the rest (by crosses I mean just that, crosses, not long lumped balls from deep in a Stoke style). It can't be coincidence that United have a manager in Fergie who had his greatest successes with this style of play in the 1990's. Fergie is 70 years old, he's not going to be employing new techniques at this point. He's not going to be paying Barcelona's training ground a visit to see if there's anything he can learn any time soon. He's a pensioner, and we all know how stubbord old folk can be! Part of United's success can be attributed to the abnormal manner in which they play, teams set their sides up to defend against the normal styles of play. If you're training all season to defend against "tiki-taka" football then suddenly having to deal with pace, strength, width and crossed hammered into the box is so alien to you it stands to reason United will benefit from that. Maybe this is part of Fergies tactical design, to be different to the rest, but I personally feel it's a happy coincidence that the stayle Fergie likes to play just happens to be a style very few others use anymore.
One thing that is for sure, when Fergie does indeed retire, whoever they bring in to replace him most certainly won't want to play in this manner. There will be such a seismic shift at United, both off and on the field, that it will be a monumental task to employ all the changes that will inevitable come, and be successful, at the same time.