The leaves on Rt. 20 are kind of annoying, they're like the marshes in DPPt except with less spots to get stuck.
And Castelia Sewers is really boring in Autumn. At least half of the area is inaccessible, including Castelia Alley, so if you haven't already gone there and gotten all of the items and beaten the trainers, you won't get to again until January. And there's not much that's new that you can access, just a female Scientist and an empty room. Very disappointing.