most logical answer would be character choice. theres lots of shoto characters in it (which normally should mean that the US should love it). the game lacks characters in the amount of characters they got (for kof standards), and in the amount of play styles they got. characters like yamazaki, oswald, anhel, gai and the likes are characters you have to pretty much specialize in to play effectivelly. a lot of the other characters in 13 are like switching from ryu to ken to gouki to dan to sakura. theres a difference but you can learn them all pretty fast. especially for vets, the norma characters are really easy to get into and learn. some even pretty easy to master in a short time. thats why kof needs more characters and a more diverse cast (like 98/02). the gimmick characters are not well represented in this kof imo.
next kof could take care of that potentially, by adding character changes (which doesnt even need to be done, since most chars are pretty much complete cept for a move or 2). something like the 96 boss team would be pretty awesome. same for oswald and yamazaki. characters like anhel, gato, hayato, silber, gai and jyazu would really spice things up imo.
problem is, the huge amount of time it takes to create characters. they really should have outsourced some most of the in between sprites to cheaper countries like korea/china and such. for instance, kyo has 6 frames during his fb, make the snkp artists do the middle frame and the final frame of the attack. let the others being outsourced do the frames in between them.
