MongoDB is a clever wrapper to a mmap()'d hash table of linked lists of BSON documents that's naive as anything but fast as long as everything's always in memory. (Well, "fast" is a strong word where Mongo is concerned. "not horribly slow".)

MOST other NoSQL solutions actually do things like treat their index differently than their bulk data so that it isn't accidentally swapped out, ruining performance. Many also do important things like supplying durable writes. (Your data may not be consistent at any one given moment, but it'll get there.)