lorax - 2:38 am on Feb 2, 2010 (gmt 0)
I assume you are referring to live text instead of text embedded within a graphic.
For me everything is design/goal driven.
Some sites need nothing but simple straight forward sans-serifs like Arial, Tahoma, or Verdana. I don't punch up the font size without some bolding because the descenders are so slight. For designs that need a bit more elegance I'll use a serif family (Georgia, Helvetica, Times New Roman) but it depends upon how much copy will be used and how wide & tall the space it will be applied within.
I will often add letter spacing to titles, bylines, headers and other attention grabbing type. I'll increase the letting to help add some white space and give the blocks of text a bit less weight. Solid blocks of text scream at me and I won't bother to read them. My goal is to soften the copy and invite the reader to read or at least skim. Then I use pleasant contrasting highlights to draw their eye into what I want them to focus on. A tasteful link color is very useful here.