[cbsmith]

> I don't know if it's the _most_ important feature, but I wouldn't build a serious site on top of anything that didn't have some sort of built-in sharding story.

You know people say this, but in practice I find that simple hash bucketing with a redundant pair works surprisingly well, particularly in the cloud. Yes it isn't fancy, but it is trivial to manage and debug, and you can do a lot of optimizations given such a clear cut set of partitioning rules.

Your problems have to get really big before a more sophisticated mechanism really pays off in terms of avoiding headaches, and often the more sophisticated mechanisms actually cause more headaches before you get there.