No matter what artwork Linux developers use for their distros, you can bet that someone won't like it.
Ubuntu's brown and orange themes might be the most frequently criticized aspect of that distro. Lots of people love the brown; lots of other people completely despise it and complain about it endlessly. I sometimes wonder if race doesn't have a lot to do with it; is the disgust over Ubuntu's brown greater among folks whose skin color isn't some shade of brown? Could be.
Canonical tries to please the Ubuntu masses, and they've rolled out some new alternative themes for the upcoming release, Ubuntu 9.04 (codenamed Jaunty Jackalope). You can check them out here. Can't wait for the flood of derisive comments at the Ubuntu forums.
Linux Mint's green theme is refreshing to many people; others find it revolting. PCLinuxOS might have the nicest default appearance of any distro, but that doesn't stop folks from expressing displeasure about it.
Mepis users have complained loudly about that distro's default appearances, leading to a huge community effort to beautify things over the past few releases. The latest release, Mepis 8.0, shipped with what struck me as the best-looking Mepis desktop I've seen to date; still, complaints about the artwork abound at the MepisLovers forums.
Like I said, I thought Mepis 8's default appearance was great; for the first time ever, I haven't changed the default colors or scheme. But that's about all I didn't change. As usual, I put in my own wallpapers, completely tweaked the panel, swapped out left- and right-arrow icons, put in a standard KMenu icon to replace Mepis' default one, tweaked the clock's appearance, chose the transparent (dark background) scheme for Konsole, and changed the mouse cursor, among other things.
And that's par for the course. Linux lends itself to so much configuration that I'll bet hardly anyone sticks with the default in any new Linux installation. I couldn't care less what the default appearance is; when I put a new distro in, I feel like I have a blank slate, and I immediately get to work personalizing the desktop.
Why anyone even bothers to complain about a distro's default appearances is beyond me. Change it. You can.