The three problems no-one talks about – almost noone, I had a good talk with the Infinispan lead  – are:
- ad hoc data fixing – either no query language available or no skills
- ad hoc reporting – either no query language available or no in-house skills
- data export – sometimes no API way to access all data
Much as I'm loving CouchDB these days, these are valid concerns if you're considering moving to a "no SQL" type database (and there are a few other concerns as well).
As always it comes down to using the right tool for the job. This is pointed out at the end of this article and in the comments, but depending on your needs you may wind up with a combination of a No SQL database for what they're great at, and a traditional RDBMS for the problems that No SQL databases don't solve.