Assuming you're going to use couchdb, is it realistic to try to just use couchapp in place of django, rails, or asp.net mvc? What would be the benefits and drawbacks?
As it stands, a CouchApp essentially require you to use AJAX for the entire application. Generally speaking, that's poor for user experience, as you are downloading the skeleton markup, running JS, usually hitting the server again, and then adding more markup dynamically. (as opposed to just downloading the finished markup up front) In addition, caching data is a different beast on the browser than it is on a server. Also, SEO is much more difficult when you have AJAX-only content.
Basically, all of the reasons you see people going against AJAX-only (ie. obtrusive JavaScript) web applications, will all hold up in this context.
On the other hand, you would be able to replicate the entire application (as well as all the relevent data) to any node that is running CouchDB. I could see mobile versions of your application running only the CouchApp while offline, since a full web server stack wouldn't necessarily be available to you. In addition, it's easy to get up and running with a CouchApp, as opposed to learning a framework like Rails, Django or Express.js.
I have high-hopes for CouchApps becoming a complete web application solution eventually, as I am a huge fan of CouchDB as a database. However, right now it just isn't feasible for me because of the heavy reliance upon obtrusive JavaScript.