Self-issued DIY-IOUs like the German minuto-zeitgutscheine
are completely decentralized. They are promissory
notes in their literal sense. You and everyone can print them at home, sign them with your name (it's notarial!), put them into circulation by buying goods and services from those who accept, while providing service yourself for all of these notes in the community.
Thus, these (and Ripple) don't have much of this centralized "administration overhead" (this will be another good point in my overview, thanks).
Without some central ledger, how would I know how many IOUs someone has given out already, or that they will be able to pay everyone back? For all I know, someone could have written millions of USD worth of IOUs, when they only earn a few 10k's a month. That system would require an enormous amount of trust
I'm sure I'm missing something, though, as I haven't had time to look into it more.