For years, we've known that Serif fonts are easier to read for body text than Sans-Serif fonts are. However this is true only for paper, or other high-resolution displays. On a computer screen, which is probably 100dpi or lower, the serifs become far too obtrusive, and so distracting.
[Hmm... has anyone seen a copy of Introduction to Geometry, 2nd Edition (ISBN 0471504580 )? My recollection is that the 1st edition uses a beautifully clear sans serif presentation.]
Italics + serifs + non-excellent resolution == destroyed legibility. For this reason, try to avoid italics on this Wiki (where the exception is ThreadMode or temporary stuff - which should probably be refactored anyway).
The default Wiki font has serifs. Try reading Wiki for a day or two in a sans-serif font.
Note that if you pick a really big serif font, enable anti-aliasing, and roll your chair back several feet, you get the readability back - at some cost to effective display area.
JakobNielsen says something about this on his site: http://useit.com/. I'll find the reference and put it here. (I (a different person) couldn't find it, though I would love to see what Neilson has to say about it).
Conventional Graphic Design wisdom holds that text set in a serif face is more readable than sans-serif text, but sans-serif text is more legible. The distinction has to do with the granularity of recognition. Serif type gives the eye more visual clues to allow words to be recognized (read) in one chunk. Sans-serif type is easier to read glyph-by-glyph.
Regarding readability and display resolution, I'd like to add that the days of low-res font rendering are gone. Fully anti-aliased interpixel rendering is available - Apple's Quartz (display PDF) engine might be the avant garde, but I expect other display postscript engines to cope up soon.
With this change in technology, most things true for paper apply for screens also. I prefer serif now, having used special screen fonts in the past.
Serif fonts aren't too bad when using Microsoft's ClearType. But I still find sans-serif fonts to be easier to read, especially for source code or other kinds of text where glyph-by-glyph recognition is important. Interestingly, I prefer reading example code with serifs, but for code that I'm working on, I prefer no serifs. -- KrisJohnson
Well, as an aesthetically oriented person, I'd have to admit that the serif fonts (not to mention Times New Roman, which is what they'll usually render as) make it harder to read. I've noticed that there are some personalities who just don't care... and others (like me) for whom the format seems to convey meaning in itself. I use Opera, so I can provide my own CSS file to specify how I want the page to render, and toggle that with Ctrl-G.