[nordsieck]

All "block chain" is is a Merkle tree. You can do all sorts of productive, interesting stuff with Merkle trees.

If you want to do interesting things with a trust-less, decentralized system, you need the mining and mining rewards.

[Retric]

There are a few file sharing applications that do interesting things with trust-less, decentralized systems, without any sort of mining or mining rewards.

Mining allows for high value transactions in a low trust environment, but lower value transactions are very possible without that.

IMO Tit for Tat is a much more interesting form of consensus building as it can be very low resource. But it's really all about the goals not the protocol.

[Franciscouzo]

It's more of a linked-list of Merkle trees.