I doubt there is any map that could not benefit from either some visual or gameplay change. For example, there could be a gameplay addition (some sort of bounce pads, for example) that would make the map more amusing for certain individuals. Some people will say the map is perfect without them, but those who want them will say it
would be perfect with the bounce pads.
Perfection is a view just as personal and subjective as imperfection...
In visual terms, there is always room for improvement. Maps that try to simulate the real world are NEVER like the real world, so something will always be missing. Architecture is a complicated subject and every map has some incoherent structure.
A map with issues can never be considered perfect, but a map with no issues or flaws isn't always perfect either. A perfect map must provoque some sort of
'I had never seen that before!' comment.
In the end, maps should be judged exactly for what they are.
You can't say a snow-themed map needs more color variation.
You can't say a map is bad because the walls are red instead of orange.
You can't say a map is bad because it doesn't have the OMG biggest shaft for vertical gameplay ever made.
etc.
For example. You can't download my maps "hexagerate" or "floating volume" and expect some "kaufmann house"
I tend to rate maps in their context, comparing them to maps that have similar themes or ideas. I do this with films and games too. You can't watch "Toy Story" and say how emotionally empty it felt compared to "The Shawshank Redemption". You can't play "F.E.A.R" and expect the game length of "Final Fantasy VIII"
sorry for the long post!