I'd love this. This could help me get more creative with my sig in terms of having words in it. Decoration purposes for posts, it would be great for mini shops, contests, quest threads and charities. ^^
I'm fine with the default font.
People already try to "dress up" their posts with post styles, which already look pretty ugly.
I would hate to see a whole post with 50 blinking images, 2 full emo song lyrics, 1 line of actual post all in the font Jokewood.
But maybe I'm being a pessimist. If they restricted it to maybe 2 or 3 fonts that weren't illegible I wouldn't mind too much.
While I'm all for personal customization, and more options for users...I rather like that there is one default font allowed...
Like others have said, there is already so much liberty taken with post styles for posts that adding yet one more editable parameter such as font type might just push me over the edge (sanity-wise)...sweatdrop