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I think it helps if there's something unique to it that sets it apart from other routes/maps. That way, every area is more memorable than just "another grassy area with trees" or what have you.
It also helps if it's visually interesting, and having some secrets that you have to backtrack later to unlock never hurts.
I think it has something to do with the variance. For instance, you could just line up a bunch of trees with the square tool, have a bunch of grass blocks lying around, and maybe a river that cuts straight through it. Or you could have a few trees placed outside of that box to give it more curves, a few curves to the river, and grass placed naturally around the area. Maybe even a few details like rocks and flowers around here and there. And maybe not just a straight path through but with a few twists and turns. Its also pretty good to look at scenic locations and study what makes them so good.
I think that a good town map has variants in all the houses, not all of them the same design or color but all the same height, and at least 2 items per map in houses.
A good route has patches of grass where it will annoy the player, give it lots of curves because rounded shapes are liked more than say, a square map...
For me it's lots of details and less empty space. Changes in elevation and lots of twists and turns if it's a route. If it's a city make buildings different heights to give it the city feel. And make the streets zig zag instead of square intersections. If it's a town break up the area with trees and elevation changes. Try to keep the buildings the same theme. Not necessarily the same color. I'm still learning how to make a good map. So practice is the best advice.