I generally find bonuses to morale are the most important things a retainer can give. Most of the + attack and defense stuff is covered by your traits, so if you build your general smart in that regard, you can turn your troops into killing machines. What I normally look for is the morale boosters, and maybe the fear inducing stuff. Though like other posters have said, once you hit the mid-game that almost becomes a non-factor.

I also make it a note to build more generals as fighters than as tacticians. Typically I have one man who is my "military genius" he is built purely for stategy, adding bonuses to units and, sometimes, the poet line for the boost to research. Everyone else gets used for the charge