#4 Not challenging for the Premier League Title
Mourinho does deserve credit for improving the Reds with it looking increasingly likely United will finish 2nd, which is a huge improvement on the woeful 6th place finish last season in the league. Nevertheless, the Premier League was Man City's a long time ago.
The ease with which City have won the title has been particularly frustrating for United fans. Admittedly they've been very impressive but United have not put up enough of a fight. They've never looked remotely like winning the league in Mourinho's 2 seasons at the club.
Furthermore, as previously stated, the playing style and number of bad performances (particularly against the bigger sides) have angered fans. United's sterile brand of football pales in comparison to the attacking riches of Manchester City.
Undoubtedly the United board will have noted Guardiola's success at City through playing attacking football and it is possible that they will eventually employ a coach willing to do that at Man United, which Jose Mourinho is certainly not willing to do.