[3pt14159]

Python (Tornado or Twisted) + Riak.

Why? You can scale very easily, write code in a higher level language that has lots and lots of libraries, have map reduce out of the box, but you don't need to really worry about your DB dying on you.

Here is what you do need to worry about in this set up: Eventual consistency. But I would argue that any web app at scale needs to worry about it. So if you are coming from a company that already has lots of users and you are writing a new app that will be in the hands of lots of people very quickly, go with Riak + Tornado/Twisted. You'll probably want to set R = 1 or 2 and pay special attention to conflict resolution.