CouchDB has some features that other databases don't have: a continuous changes feed, REST interface, master-to-master replication, a web interface to the data and management (Futon). We need those features (Yes including Futon. It is a feature because it lets us quickly prototype and debug. It makes a black box that you drop your data in transparent).
But we not using to for large datasets. We are using it mostly for configuration, and setup. There is a custom clustering setup built in its m-m replication and changes feeds.
But I agree that for IO scaling and large data sets Riak would be a top choice. But there are other contenders to look at as well: Cassandra, BigCouch and the upcoming Couchbase Server 2.0
Every database is different, but that doesn't mean there are a lot of really unique features. The unique feature of CouchDB is the way it does views, but that doesn't mean you can't' do views (in fact, in my opinion, better) in other databases.
Continuous changes feed- you can get this with Riak and more importantly you can get a feed of just the relevant changes. Plus you don't need this in Riak the way you do in CouchDB because Riak already has distribution built in.
Master-Master replication as done in CouchDB is inferior to the turely distributed database that Riak is. (Eg: its not replication, it is distributed itself.)
Web interface to data management-- futon was a lead here but there are several tools for Riak that cover these bases in my opinion.
I think CouchDB as a configuration database is an excellent job.
I looked at BigCouch which is taking the Dynamo Ring concept and applying it to CouchDB which is a good solution for couchDB (in fact, they should build it in to the core) ... but that's also what Riak is built from the ground up to be (a dynamo ring.) Cassandra is a different animal and I can never figure out what Couchbase is going to become.
But we not using to for large datasets. We are using it mostly for configuration, and setup. There is a custom clustering setup built in its m-m replication and changes feeds.
But I agree that for IO scaling and large data sets Riak would be a top choice. But there are other contenders to look at as well: Cassandra, BigCouch and the upcoming Couchbase Server 2.0