Everyone keeps mentioning Mat as the biggest problem, but I have to be honest, I don't see it. I am definitely a lazy reader, especially with WOT due to all of the fluff, but still didn't see any red flags with Mat's character.
By the way, was this problem evident in only AMOL or also in GS and TOM?
But I don't really remember much from the previous two books aside from the one note Mat wrote to someone (can't even remember who it was for) that was horribly misspelled as though he was barely literate. Even if BS for some reason believed that Mat himself is an idiot and skipped his schooling or something, surely at least the multitudes of memories in his head would make him capable of writing properly? Those people were generals and commanders after all, so they had to read and write reports, and be of well above average intelligence.
As for AMOL, it isn't too specific, but if you notice, Mat is frequently described as sauntering rather than walking. Which isn't a terrible thing by itself, but it shows the way Sanderson perceives Mat, and that attitude and tone simply permeates all the Mat segments of the book. Sorry, I can't get more specific than that without doing a reread of the book while on the lookout for specifically this.
EDIT: mentioned this in my response further down to Nighthawk, but Mat leading a charge during one of the battles (it was in the battle Egwene's armies were participating in) was completely unlike him, and poor military judgment to boot.
*NM*