I think there's nothing wrong with plain text links. I do like some indication they are links though. The standard underlining seems to work well for me, although some people consider that ugly. If the underlines are removed, I generally like the links to be bigger, and to have a mouse-over effect to highlight them.
I have a similar opinion on link colors too. I sometimes find it useful to keep track of where I've been. But again, some people find it looks ugly after the user has visited a few random links, and so the links on the page are now a random mess of two different colors. For a few well traversed links, I think it's fine to have no distinction. Something like main navigation elements when there are a small number of main sections, or users are likely to be very familiar with all pages. For something that links to hundreds of different articles though, it's nice to know which ones you've read. I do feel some attention needs to be given to both link colors though. You do want reasonable contrast with the background color or image for both visited and non-visited links. It can be easy for a web designer to forget about one of the two.
