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.