I'd like to explain the emblems I got in use right now, mostly thanks to the tango generator icon theme, and how this method should be the gnome standard and really a default for the human theme.
What emblems to include
-----------------------------
It's simple, when you click the applications menu, you'd see icons like "office", "development", "sound and video" (aka multimedia) , the first good thing about the emblems made by tango generator is that it uses those icons as emblems. So, for folders you can really categorize them correctly.
Another great emblem is the "star" tango emblem, firefox, gmail all are starting to use these stars to mark the more special things , a little star emblem helps me determine what things are priority, much better than such clock emblem.
So, the list of emblems to include would be:
special (the star icon)
trash/dump (This uses a mini trash icon)
multimedia (icon currently in use by sound and video category in applications)
development (The menu one, for example in tango it is a construction helmet)
art (The one used by "graphics" category in the menu)
office (idem)
system/gear (Again, from the menu, optional, but I found it useful)
download/web (I guess the one from the menu works)
fun (A smiley)
danger (A bomb icon probably)
urgent (I guess some people would want to use this instead of starred)
money (A human themed $$ sign could work)
[....]