I would second Mathias' thoughts in that this does feel more bigtable/column oriented than traditional dynamo. Dynamo and its derivative, Riak (not from amazon), make no attempt to determine data types or schema in any way. The fact that dynamodb can do range scans and has various counting features leads me to believe that it is more hybridized along the lines of Cassandra. Either way, it is a welcome addition to the nosql toolset that developers have to chose from today. I will certainly look at it more closely.
Search and Indexing were addons to the initial Riak product that were born from user demand. Only recently (1.0 I believe) was Search integrated into the main Riak distribution. Riak has no capability of updating values in place, ie. a partial update and no means of ordering or pagination. Some of these features are available via mapreduce queries or Search/Index as you noted.
I'm by no means harping on Riak. I actually use it on a number of projects. But reading about DynamoDB's capabilities does not conjure Riak, it conjures Cassandra and HBase.
