Good question -- as far as new, mature NoSQL solutions on the scene, I just became aware of OrientDB which sort of baffles me with it's functionality. It looks like this amazingly functional blend of MySQL and NoSQL:
http://www.orientechnologies.com/orient-db.htmOther than that, I actually think these solutions have been stabilizing exactly because of what you say: innovation is slowing down/stalling.
1-3 years ago the cool thing to do was store data different ways, now that we have all these solutions that people are ready to use in production, they are demanding more and more secure/safe functionality from them.
In the last year Redis added the append log and flushing to disk. CouchDB rewrote the replication code in the last release and has always had a wonderfully redundant and safe file mutation model (can can copy the DB file while in use and still get a safe snapshot) and MongoDB has been responding aggressively to crashes and corruption since 1.7 after all the single-server durability fiasco around 1.5/6 that had everyone up in arms.
These data stores are really brilliant pieces of code with some wonderful deployments to prove their worth.
There is still work to be done, sure, but I am not aware of glaring deficiencies in these systems like I used to be a year or more ago where you could point at "Oh, the XYZ bug might get you" -- that just doesn't seem to be happening anymore.
I don't know a whole hell of a lot about Cassandra (I am one of the few humans that still doesn't grok the data model easily) but I remember data recovery bugs from a year ago in the issue tracker that all got knocked out to the point that 1.x is looking like a really awesome release for them.
At this point, I think it just depends on what you need.
