As you and others have pointed out, hosting + SSDs + synchronous replication across availability zones counts for a lot. If DynamoDB lives up to the hype, it could be a huge step forward in the world of "don't have to think about it" data storage.

DynamoDB does have at least one significant feature not provided by Riak -- range scans. This makes many common access patterns much easier to implement efficiently. Still, as you suggest, there don't appear to be any fundamental technical advances here. The advances are in the service model and operation.

And there are, of course, many limitations. (Just to name a few: items -- i.e. rows -- can't exceed 64K; queries don't use consistent reads; seemingly no atomic update of multiple items.) It's miles ahead of SimpleDB, but still not nearly as flexible as many of the existing NoSQL databases. If Amazon lives up to past performance, they'll make steady improvements, but slowly.