All Guan apologetics go here. After a long period of being indifferent to Shu and considering the generals overrated, I'm coming back around to seeing Guan Yu in a very positive light. What sparked this is how he was said to be loved by his men. Now just how far this went, I'm not sure, but I'm not going to count it out just because it was a rare thing.
Indeed, few generals in history can be called "loved" by their men. Wellington had the respect of his men, but they didn't love him. Veterans will tell you that they were proud to serve with Patton, but they wouldn't say they loved him when they did.
The few who I can think of that it can surely be said are:
Alexander the Great
Gustavus Adolphus
R.E. Lee
and, of course, Guan Yu
That alone boosts his image in my eyes because a general who can elicit such loyalty from his men has mastered the art of leadership, which is no less important than being tactically proficient.
Then there's the fact that he was a great man of learning. Guan was the true scholar: reading and becoming a master by his own standard. He was a trailblazer, and if the legends are to be believed, inventive.
Assuming that he was willing to risk all for his beliefs -- whether it was killing a ruffian to protect a girl, or dying for his country -- I think that Guan Yu's merits off the battlefield alone stand far above the fatal flaw of arrogance.