Yes and no. A block chain is just a distributed ledger (database). “Currency” or “usage tokens” are built-in to prevent spamming the network with nonsense ledger entries. Once a currency introduced, we then must deal with the question of how to distribute the currency and there are many options for this. If you don’t want a currency, prepare to have your network massively DDoS’d.

• Could the work needed to maintain the blockchain be done based simply on reciprocity of proof of work? For example, if bittorrent required users to seed as much as they download. Or, in the case of a distributed dropbox, users would have to provide as much storage space and processing power to the network as they request from the network. Is it possible to require reciprocity of work without those work credits becoming a currency?

The proof-of-work in Bitcoin and other cryptocurrencies is work that can be easily verifiable by other network participants. Any proof-of-work based on information external to the blockchain could be easily gamed. What is the aversion to a currency?

• Bittorrent Sync is an interesting project that allows users to share selected folders and files without an intermediary. What advantage does the blockchain have over this? Perhaps this functionality can be expanded and combined with the blockchain if the blockchain is the group contract and consensus piece to the puzzle.

Some people are already looking at ways to merge these ideas to make a P2P browser or social network (Syncnet and Twister, respectively).

Ethereum and Ripple are, IMO, the most promising next-gen protocols in this space. If you don’t like the methods these projects are using to distribute their native currency, you can always fork the project and try another way.