For items that are not simply tag searches, the contrast between the colors of a visited link and a non-visited link are very low, making it difficult for me to determine what pages I've visited.
For example, on Stack Overflow, this is what I see:
It's even worse on Meta Stack Overflow:
It's also on all of the beta sites:
I appreciate subtlety in design, but this is one of those areas where it's a major hindrance.
I'm currently viewing this in Chrome 24.0.1312.52 m on Windows XP SP 3.