Well I think your tileset looks very nice :3c
I'm not sure what they were thinking exactly, but the only thing that I can think of would be the saturation of your basic buildings (PC, Mart and Gym) along with a few other things (roof of the well, your grass) as they can could be considered 'faded' in comparison to the bright colours the rest of your tiles are generally displaying. Also a number of your tiles have very deep/ noticeable borders (Your buildings and large trees) while the borders of others (Smaller trees/ bushes, tuffs of grass, the well's roof and even your mountains) aren't as impressionable. You could remedy that by using a deeper colour as their borders where applicable. Its being a while since I tried to critique tiles so I feel a bit out of it so sorry if I'm a bit over the place.
Holy moly we haven't talked in a year ;_;