The festivals I reall like are those with a distinct personality--Whitby Week for trad; Mystic Seaport Sea Music festival for a strong theme. Lots of workshops--and I don't mean mini-concerts. Reasonable separation of venues (it's painful to have a background of mixed Cajun and Bluegrass from nearby tents when you're trying to hear a quiet ballad.)
Adequate locations for impromptu picking and/or singing gatherings.