One of the annoying things that some websites do is make it impossible to see the difference between visited links and unvisited links. This is not just irritating, I maintain that it is somewhere between a bad idea and a terrible one for real blog usability; how bad it is depends on how densely interlinked your writing is.

To summarize, for real blog usability you want to encourage people to explore your site; having landed on one page through a search or an inbound link from somewhere, you want them to keep reading more of your content. The problem with not being able to distinguish between visited and unvisited links is that it discourages exploration by causing visitors to waste their time.

If you interlink articles a lot, people are going to take an unpredictable branching path through your work. Almost inevitably there will be multiple paths that wind up at the same place (because you refer to the same article in multiple other articles); the more dense your interlinks, the more such path overlaps will exist. When visited links can't be distinguished from unvisited links, visitors can't tell whether what they're thinking about clicking on is something new (and interesting) or something that they've already read where clicking through the link will just waste their time. This inability to tell whether you're about to waste your time is a discouragement from clicking all links, and it doesn't take much discouragement before your visitors learn better. Good for them but (presumably) bad for you.

The clear conclusion is that you want visitors to be able to tell if they've read something before they click on the link, not after; you want exploration to be as risk free as possible, not randomly risky and time-wasting. Browsers give you this almost for free, provided that you do not force visited links to look just the same as unvisited ones.

(If you feel ambitious about CSS stylings, the logic here suggests actively de-emphasizing visited links compared to unvisited ones instead of just having them in a different colour.)

Sadly I have run into any number of websites with interesting, densely interlinked content that committed this mistake. The result is very frustrating; they had fascinating stuff, but they wasted too much of my time in getting to it.

(This is the same mistake that pushed me towards Reddit over Digg, way back when, and the reason was exactly this; Digg made it risky for me to click links and Reddit did not. I note that Hacker News has done Reddit one better by making visited links subdued, not just a different colour.)