I love and use Redmine, but there's one thing that I find quite annoying -- readability of the default theme. Yes, you could say that I should create my own theme or use someone else's theme -- and I do, but since most of the people don't and since redmine.org is the site that represents the project I think it is beneficial to work a bit on improving the readability. Also, it makes it easier for theme designers to create new beautiful themes by editing the default one.
1) #content div should have a wider left margin
2) default text font could be bigger (Wiki, Roadmap etc.)
3) items in issue list should have a bigger font size and padding, currently it's a bit pain to read and it looks like the most important item in the page is the filter box and not the issues themselves; also applies to the list in roadmap
4) in download section (which I guess is just a simple wiki page) the headers should have bigger top-margin, so people would see a clear separation between sections (and now the top and bottom margins are more or less the same -- which is unintuitive).