To the last couple commenters:
“but for the average user…they don’t care.” I disagree. In fact I think the average user cares a great deal how their desktop looks, or, at the very least, their opinion of an operating system is informed to a large extent by how the desktop looks. They of course don’t consciously examine pixel metrics and font hinting and gradients, etc., but they most likely notice them subconsciously, contributing to an vague un-ease with their computing environment. All these little things add up to make a bad impression.
“It is trivial for the user to change everything to the way each user wants it.” I also disagree with this. While it’s trivial to change among the pre-installed themes, knowing where to go for new themes and what to do with them once you’ve got them is not “trivial” — it’s not difficult by any means, but it’s something you have to look up the solution for. And it’s something you’d only attempt to do after having decided to live in Ubuntu for a while — a decision you’re less likely to make if you’re promptly turned off by the aesthetics of it.
There is no substitute for the experience of the first five minutes of using any piece of software, and if Ubuntu is going to convince people to give it a chance, it needs to be more attractive by default than it is.