One of the single biggest ‘critiques’ about web sites and blogs revolves around cosmetics. In our opinion, a single font should be used throughout a site; the eye does not track content as well when wanting to assign ‘meaning’ to the font changes. In turn, this causes the reader to jump around on the page.
Most desktop publishing articles/books suggest that no more than two fonts should be used with a maximum of 4 font size changes in a ‘publication’ – read that as ‘web site’ or blog page. In contrast, it is acceptable to use a little more diversity in the number of ‘changes’ that appear in a particular page.
On the extreme side are those who advocate no more than 2 changes per page and on the lenient side of the debate are ones who suggest that up to 4 changes per page is acceptable. But even the latter suggest that content readability demands changes due to need for emphasis or to separate divergent ‘subjects’.
As an aside, when laying out a magazine spread, the lenient side is more acceptable than the extreme when considering font size. Generally speaking, readers won’t be happy with pages that include font type changes.
Note that font type (‘family’ in http parlance) refers to Ariel, Times New Roman, Courier, etc. Size refers to 8 point, 10 point, 12 point, etc.
Also note that text color can also be used very effectively in place of font changes.