What's the concern for lock-in here - how difficult with DynamoDB be to migrate away from? Amazon seems to be increasingly catching the App Engine Syndrome, though to be fair, it's been mostly network traffic-style functionality in the past.
In Simple Hash Key mode, it is just a key-value store. All the logic is in your app and/or supporting libraries, not in your DBMS.
In Composite Hash Key mode you also get limited range queries. These are supported by any DBMS that indexes its primary key via BTrees.
DynamoDB is cutting the features to the bone. This makes it easy to migrate out of DynamoDB and, at a first glance, hard to migrate into DynamoDB. Particularly hard to migrate from a RDBMS with complex schema/index/trigger support.
The beauty of it is that complex features can be built as libraries as the needs arise. The next five years look very exciting. Hats off to Amazon.