"Mongo uses (auto) sharding as our path to scalabity (sharding is in alpha)"

it's my understanding that mongo currently only shards primary key lookups, not queries. it'll be interesting to see how successful they are in grafting a full distributed layer over top of it. generally the merge layer kills you because you must query all nodes to support global indexes. that is not to say that its impossible.

mongo has some wacky stuff in the on-disk file format which is kind of interesting, too.