"Based on distributed Solr" might be an over simplification. It uses Solr as its indexing engine, but really, that engine could be any single-node indexer... including single ES nodes. Basically, Yokozuna adds real grownup distributed systems computer science to the OSS distributed search space. http://docs.basho.com/riak/2.0.0beta1/dev/advanced/search/
I think the biggest pluses for Erlang as a language are that fault-tolerance (Erlang "let it crash" + OTP supervision trees) and distribution of tasks (process interact via message passing, be it across procs or nodes) are built-in language features. They aren't libraries or afterthoughts. This subtle shift manifests itself in all Erlang code. There's rarely a fear that some esoteric library you want to use has a memory leak because it's keeping state in some static hashtable, or that semaphores are used incorrectly causing a race condition.
In a sense, yes. Luwak was just basically plain-old Riak with some support for huge files. CS is a full solution for storing large assets, along with multitenancy, role management, reporting, monitoring and pretty much anything else you'd expect from a cloud store like S3.
I hear that. Riak docs used to famously suck-out-loud. It wasn't that the information was bad, it's just that no one had the time to devote to making them clean, consistent, or easy to navigate (ie. Info Arch).
Now we have some of the best NoSQL docs[1] around (clearly there's still more to do). A nice side-effect of cleaner navigation is that PRs to our repo[2] have increased dramatically.