I dunno, perhaps it's the way the grass looks in tandem with how awkward / barren that island does.
Every time I see the release of a new set of games, I always hope that the grass looks nice, but it always seems to look really bad in comparison to previous grasses. I don't get what is so hard about making grass that looks decent; animal crossing is able to do it (although that game is generally quite nice visually) and windwaker was able to do it years ago; why can't Pokemon make the grass be at least somewhat realistic looking? I mean seriously, would it kill them to make maybe 10 different models for grass so that routes at least look natural? It's ok for a texture to repeat in a sprite based game, I guess, but it looks really, really bad when you see a repeated string of those ugly grasses.
Jumping off of your Pokemon while using Surf looks really off as well (surf looks awkward in general... looks like they copypasted the wave thing from bw but did it rather shoddily). Again, animal crossing manages to make this type of action (getting out of water) look nice; it looks really strange here.
The battle scenes have always been really nice, though, so I have nothing to complain about there.