I did not see a specific place for Garland discussions, so I hope this is the right place.
First, it is AWESOME. These are detailed issues here, but I know everyone wants to get issues like this right.
A few things about color and links that I think could cause usability problems.
1. Visited and unvisited links always have the same color / decoration. Should we have 2 settings for link color? I know, you can hack the .css - I just did it in 5 seconds, it was not that hard. But I am not comfortable with the built-in assumption.
2. Links are not underlined. Again, easy to hack. Should it be a setting?
3. There is a single link color but it has to work on 3 different backgrounds (center space / usually white, left/right sidebars and breadcrumb area/usually the darkest of the 3). It will be very difficult to get the CONTRAST right to have a single link color work well on all 3 backgrounds. For example, in the Belgian Chocolate setting, the link contrast is fine for center and left/right but not good at all for the breadcrumb. I think the breacrumb will be unreadable for some users - the link and its background do not have enough contrast.
Not sure if the breadcrumb needs it own link color or what.
We cannot prevent people from making unusable instances of Garland, but we should also not make it unusable out of the box.
Even with these issues, I would not automatically add in loads of more Garland color settings - the Garland set up has to be usable too. Perhaps the best way is to leave as is and then add some guidance on how to hack the CSS that results.