What Carlos said, here's a longer list of reasons:
- I'm a Liverpool fan and the rivalry with them is intense both socially and for footballing reasons. Man Utd. also have historical/currently big rivalries with other extensive fanbases, such as Leeds, Arsenal, Man City, Chelsea etc. so you'll find a lot of football fans with an axe to grind.
- Alex Ferguson is a brilliant manager, but his arrogance, petulance and lack of respect for authority figures symbolises the 'we're Man United and we'll do what we want' attitude of the club.
- Gary Neville, prime tosspot
- They're not unique in this but their fans are disgraceful at times. Rarely Mancunian themselves, they crop up all over the country, the worst kind of glory hunters, attaching themselves to a club they have no social or familial connection with. This I do not necessarily have a problem with, but when someone with a Southern accent loudly proclaims how 'his' team are great despite not knowing the names of any of the players it's terrible, and they're the worst for it
- Linked to above: large Irish fanbase. Irish plastics are nobbers to a man.
- If you're going to be champions, do it with class. They don't. Well-known for surrounding referees, dirty play and bad sportsmanship when they feel they can get away with it.
- We're all jealous of their consistent success and great footballers.
Also, Barcelona are a very likeable team in some aspects. Astonishing play, great players, Guardiola (manager) is a gent, have a largely home-grown team. They can be petty at times but not many neutrals would favour Manchester United, that's for sure.
That's enough de-railing now. Sorry.