As part of the Astoria design process we scanned through many topics, some of them are straightforward, some are hard but mostly mechanical, but there are some that become interesting, fundamental aspects to address.
I found the problem of concurrency control over REST interfaces very interesting to explore. The problem is actually well addressed in HTTP so it wasn't that we had to invent a bunch of stuff. It was more about learning what was out there, reading about good/bad stories, and then mapping it to our model and see how it fitted.
In the end we landed on what I think is a quite nice place. If you're interested, check out this write up in the Astoria team blog on the topic.