Decentralized product inventory (instead of simple orderbook)
I think Ethereum is ideal for this.  As you say, it's not ready for prime time yet. But I think it offers the most flexible toolkit for managing orders in a truly decentralized way. You can create and manage arbitrary database structures that are secured on the blockchain.

I wrote a toy digital artwork market using Ethereum -

http://robmyers.org/2014/07/24/ethereum-art-market/