Disclosure: I wrote a product called Citrusleaf, which also plays in the NoSQL space.
My focus in starting Citruseaf wasn't features, it was operational dependability. I had worked at companies who had to take their system offline when they had the greatest exposure - like getting massive load from the Yahoo front page (back in the day). Citrusleaf focuses on monitoring, integration with monitoring software, operations. We call ourselves a real-time database because we've focused on predictable performance (and very high performance).
We don't have as many features as mongo. You can't do a javascript/json long running batch job. We'll get around to features - right now we're focused on uptime and operational efficiency. Our customers are in digital advertising, where they have 50,000 transactions per second on terabyte datasets (see us at ad:tech in NYC this coming week).
Here's a performance analysis we did: http://bit.ly/rRlq9V
This theory that "mongo is designed to run on in-memory data sets" is, frankly, terrible --- simply because mongo doesn't give you the control to keep you in memory. You don't know when you're going to spill out of memory. There's no way to "timeout" a page cache IO. There's no asynchronous interface for page IO. For all of these reasons - and our internal testing showing page IO is 5x slower than aio; the reason all professional databases use aio and raw devices - we coded Citrusleaf using normal multithreaded io strategies.
With Citrusleaf, we do it differently, and that difference is huge. We keep our indexes in memory. Our indexes are the most efficient anywhere - more objects, fea. You configure Citrusleaf with the amount of memory you want to use, and apply policies when you start flowing out of memory. Like not taking writes. Like expiring the least-recently-used data.
That's an example of our focus on operations. If your application use pattern changes, you can't have your database go down, or go so slowly as to be nearly unusable.
Again, take my comments with a grain of salt, but with Citrusleaf you'll have great uptime, fewer servers, a far less complex installation. Sure, it's not free, but talk to us and we'll find a way to make it work for your project.