MongoDB: what are the technical limitations preventing document-level locking?

I use MongoDB pretty actively and I've been wondering this for a while. I recently did some research as to why MongoDB can't have document-level locking in its current state. The only thing I managed to find, besides references to the intra-collection locking ticket (https://jira.mongodb.org/browse/SERVER-1241, btw is "intra-collection" the same as "document?") was some obscure forum post about b-tree indexes not allowing concurrent writes.

Obviously, not having a global write-lock would increase write (and consequently read) performance by orders of magnitude, especially for in-memory data sets.

Does anybody have insight as to what the technical limitations are here? I'm really curious!