riak does support automatic sharding and enables you to tune your CAP paramters. Getting nodes in and out of a cluster is way easier with riak. While riak does also support map/reduce, generating something like a couchdb's "views" will probably end up being a problem since listing all the keys of large buckets (needed for a view of all your data) is a really expensive operation in riak. Also: couchdb supports incremental views, so if the view is fixed, you'll only need to update the changed documents. My main problem with couchDB was that in systems with a lot of changing data, the compaction was too slow for me. I actually changed data faster than the compaction could work on. Also: you have to compact that stuff yourself by sending an explicit http request.

would you be willing to write up how your writes were outrunning the compactor in a little more detail in either a blog post or in an email to the apache couchdb dev list?

we've been talking about possible improvements to the compactor and having a solid use case that outruns it would help focus that.