I think the comparison to InnoDB is apt; many of the same shortcomings/features are there, as well. MyISAM/mongodb both have table/collection locking, no trans

I think the comparison to InnoDB is apt; many of the same shortcomings/features are there, as well. MyISAM/mongodb both have table/collection locking, no transactions, make no ACID guarantees. WiredTiger/InnoDB by contrast introduce row/document level locking, transactions, and ACID. Incidentally, this is why we moved to TokuMX over vanilla Mongo; it provides similar benefits, and is production-ready today. The disk usage is massively improved, document locking means that I practically never have to worry about lock contention (and if I do, it's usually a sign that I have something messed up in my app, rather than something wrong in my DB), I get guarantees about durability, they've even done some heavy work to fix Mongo's election protocol which fixes a number of outstanding issues in failover scenarios. With those features in place, it's actually a decently respectable DB; if WiredTiger can deliver a similar level of improvement, it'll do very good things for Mongo as a DB and a company.