1. It solves an extremely practical problem (spam) using a very simple idea (provide a value that when appended to a message gives a hash value within an acceptable target range).
2. It can be understood by non-experts who simply understand (or accept) the one-way nature of cryptographic hash functions.
3. It can be implemented manually, given a working hash function.
4. It sat around for a decade in obscurity until someone dusted it off to build a system most thought was impossible.
https://bitcoinmagazine.com/articles/genesis-files-hashcash-...
1. It solves an extremely practical problem (spam) using a very simple idea (provide a value that when appended to a message gives a hash value within an acceptable target range).
2. It can be understood by non-experts who simply understand (or accept) the one-way nature of cryptographic hash functions.
3. It can be implemented manually, given a working hash function.
4. It sat around for a decade in obscurity until someone dusted it off to build a system most thought was impossible.