I've just installed linux mint 14, and love it. But one thing I didn't like was the default theme and fonts. I think if they were refined it would give a better first impression of mint!
1. The window decoration in the default theme has jagged edges. Cinnamon's window manager *does* support properly anti-aliased window borders, but the window decoration theme has to be properly designed to take advantage of it. For example here is a screenshot of my linux mint 14 desktop using the greybird theme. As you can see the corners are very smooth compared to the default mint theme: http://i.imgur.com/jEVrV.png
2. The bright green menu highlights on the default theme are a bit much. In addition, a selected menu item has the bright green highlight and white text causing readability issues, and just looks grating. I like the idea of a nice green theme, but it should be a bit more subtle.
3. The default fonts just aren't very good. Sans isn't a very nice looking interface font, there are plenty of superior interface fonts available (for example droid sans, liberation sans, ubuntu font). The screenshot in # 1 shows droid sans + liberation mono for the terminal font.
4. The dark nemo sidebar looks nice, but combined with the default icon theme you end up with a few cases of a black icon on a black background, which isn't ideal (for example the home folder icon)