The deficiency with AND queries due to term based partitioning reminds me of a hack I did a few years ago trying to build a basic IR system on simpledb:

http://codingthriller.blogspot.com/2008/04/simpledb-full-tex...

Of course, I was backed into this corner by the various constraints at the time, and this was just a fun hack. It sounds a bit precarious to consider using Riak for text search, at least compared to something like Solr, if it truly has issues with the basic bread and butter of search like sorting by recency and conjunctions. (of course I am only going on what is in this post, I have no experience with Riak)