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.