I find it a better course of action to work on properly understanding your data problem and then evaluate the options available instead of just sticking with the stand-by solution ("Well, we've always done x!").
Just like with anything in technology ... there is going to be pain as you escalate the level of complexity and what you are trying to accomplish.
I disagree, based on experience. On the surface, MongoDB did solve our data problem, and we thought we sufficiently researched the technology that it would work, even attending multiple MongoDB conferences and talking to 10gen employees.
The issue was that underlying architectural decisions in MongoDB ended up biting us and limiting us rather significantly. It could be argued that this is because MongoDB is a rather new piece of tech (I disagree, I think MongoDB is fundamentally flawed, but it doesn't matter in this argument).
Because of our experience, going with the standby IS the best choice, until you _need_ something else. Riak was a change necessitated by its fast growth such that horizontally scaling was necessary when you're in an environment such as EC2. Ignoring MongoDB specifically, the IDEA of MongoDB simply isn't correct here, a Dynamo-style K/V store is the correct option, and Riak happens to be a fantastic one.
Just like with anything in technology ... there is going to be pain as you escalate the level of complexity and what you are trying to accomplish.