Add this to the very bottom of StackOverflow's style sheet (or use stylish to test it)
a,div {-moz-border-radius:3px;-khtml-border-radius:3px;-webkit-border-radius:3px;}
It instantly will soften all the hard edges for almost all the boxes on your site that drive my eyes crazy. The tags, etc. instantly become softer.
Unfortunately there are still a few items that don't have borders so it won't work on them (like the horizontal menu at the top of each page) but it's still a great improvement. This is subtle, but it makes a difference.