I feel like PoW is an underutilized spam prevention technique, since most clients don't support it, and most relays don't require it.

Now that pagerank is a thing, I think public relays could do a lot to minimize spam by simply requiring PoW for anyone below a pagerank threshold, but only if more clients supported adding PoW to notes.

Maybe it's a chicken and egg thing... Relay operators are reticent to add PoW requirements since so few clients support it, and client devs are reticent to add the option to include PoW since no relays are requiring it.