It's a distributed multi-node key-value store. They built it using Erlang (I almost consider that, alone, a "feature").
(Also, if you like S3, keep your eyes on Basho over the next few weeks).
Well, that's the filter I use when looking at anything. If its not written in erlang I spend a lot of time trying to figure out exactly how it isn't really actually a distributed system. Usually I find out that it isn't.
If something says it is parallel this, or concurrent that, that is when I filter it - thus far Erlang's concurrency model has been unmatched (in my limited experience) by any other language I've used for efficacy and simplicity.