In the accord thing, these are the things i notice and don't like....
Cheap looking rims. I don't care if they were cheap as hell or Rays, Work, etc. If they look like they were sold on sale at pep boys, it makes your car look cheap.
Excessive body "add ons"-A lip kit usually looks good. But even a lip kit that doesn't flow ruins the look. Add a bunch of lights, visors, flares, with no concern that one part matches the look of the others and it cheapens your car.
"JDM" mods-JDM exterior/interior mods usually look fine on cars with a japanese equivalant because they were made by Honda, or high end tuner companies. Since we don't have much in that area, slapping on universal JDM stuff from other cars looked, well slapped on.
DIY mods that are obvious you did by your self. If it has imperfections or you did it because it didn't cost you anything, without fitting in with other things on your car, i probably won't like it.
"cookie cutter" mods-I don't like when people have a checklist of mods they feel they have to do to have a nice car. Or when people have 10+ mods and none are unique. Modding cars is all about personalizing them so if your car looks like 5000 other cars, it's really just a statement of not having much personality in your car.
I guess my main thing is taking a perfectly good car a cheapening it by either cheap, tasteless, or uncessary mods that do nothing for the style of the car.