i guess for me it's that I don't associate "userhead" with "favicon".For LiveJournal and dreamwidth they aren't the same thing, for example. And I see why they are for all of the other sites -- it's not like we are going to make site-specific userheads for all of the other sites out there -- but even so, my brain parses them differently. Favicons I barely register except in those rare times when I am looking at my tab bar and trying to figure out which tab I'm looking for (multiple times a day, I admit, but it's peripheral, not front and center). When AO3 changed theirs, there was a moment of WTF, but it was never in the center of my noticing. But userheadsare right there in the main body content, very much in your face.