My number one level making rule has always been that detail should make the level look better.
Detail is NOT I repeat NOT, filling your level full of different tiles in an attempt at having detail. Detail is adding final touches to neatly made levels.
I always argued with birdbird because he made random looking jungle-type levels and he said it was because "land isn't supposed to be straight and clean" but he did the same kind of tiling inside houses, in towns, on man-made paths, etc.
It's okay to get a little more wild on outside forest and swamp type levels.. but in towns there should just be grass, some bushes, and nice man-groomed looking scenery. Not wild trees growing halfway into your building.
Pathways that are near towns should be a bit straighter and more man-made, then the weird jungly natural pathways found in forests.. since, the paths were probably made by man and not just natural wear and tear.
All paths are generally made by man, since man is mainly the only thing that travels through the same area, and generally takes the same route (which would be the easiest route) meaning a path would form.
Personally I think the level could use a little bit more, but to say that detail means you have to have crazy looking paths, trees everywhere, and swamp/messy grass tiles mixed in all over the levels is insane.
Of course this is all just my personal opinion, other people have different views on "detail"