Easy horizontal scale path:
It's HTTP. Guess what there are already a lot of and well understood? HTTP load balancers and caching systems. It has master-master replication built in. Your database logic is in a design document, which is just another document that can be replicated across nodes.
With support for Android and iOS you can now take the same database and use the built in replication to take it up and down from mobile, desktop, and server. Awesome.
Anything can talk to it. You don't have to have a middleware piece to talk to it for many web application scenarios. This can greatly simplify things.
You can have a lot (I mean a lot) of systems monitoring for changes and apply filters to get just what you want.
Concurrent at it's core:
Thanks to Erlang and the ideas of MVCC you can have many connections to the database with no worries. I think I have read that some test cases found that you would run out of IO bandwidth and ports before CouchDB would stop responding. It might be slow (probably have time outs), but it's up and talking. Write speed is limited since it's serialized on a node, but in either case you could load balance that.
Now this wouldn't be completely fair I suppose if I didn't list some of the pain points:
Documentation is a bit spread out.
You really have to read the book and mix in the wiki and blogs.
Auth/Auth system is pretty primitive:
Sounds like there is some ideas around to improve this and it'll probably get you through most cases. If it doesn't get the job done of course you could just add a middle layer to manage these things. Using other database technologies you would probably have a middle layer anyway that managed auth/auth, so probably not that much of a pain point.
You pretty much have to study CouchDB for a little while before you grok it. There aren't a lot of features to make you feel at home when coming from a traditional database technology. Biggest one obviously is that any real querying is done via predefined map/reduce views. These are computed incrementally and have no side effects. It's hard to get a decent mental model of using documents and map/reduce.
There are probably more for both the pro and the con list but that sums up what I tried to hit recently.