There is plenty of talent on the team but it's not exactly well distributed. Di Maria, Rooney, De Gea...world class or near it anyway.
Great teams (or top 4 teams) aren't so unbalanced and that's something that I actually agree with LvG on. Looking back at the late SAF years, the quality was much more evenly distributed. VDS was one of the best goalies in the league. Rio and Vidic were arguably the best CBs in the league. Scholes, while aging, still controlled a game like no one else. Evra was one of the best LBs in the league. Rooney, Tevez, Ronaldo, Berbatov and, later, RvP all played a role in winning between 2008 and 2012.
But they have a man who was bought as a replacement for Cristiano Ronaldo playing right back! Smalling, Jones and Evans are average at best and even when they are good, they are inconsistent. Shaw is still very young. Rojo, I think, is one of their better defenders but his talent isn't enough to offset the others. Fellaini is OK for what he is but is he an elite attacking mid...the position he's been playing in?