My personal "big picture" critique of MongoDB is that I see it evolving into a SQL database with a different syntax. It is a strongly-consistent, distributed-through-replication-and-sharding system. Additions to 'traditional' databases, like Postgres' HStore or MySQL's HandlerSocket show that many of the MongoDB differences are not fundamental.
Much more interesting to me are systems that do something fundamentally different. e.g. explore different parts of the CAP trade-off, like Cassandra. Or systems that are built around the idea that data no longer lives exclusively on a server, like CouchDB.
I think I disagree with you on a few points. MongoDB might be adding features of SQL databases, and heading in that direction, but they are still missing joins, and that's a crucial part of relational databases.
Also, I'd clarify a little by saying it's trying to be a strongly consistent, distributed through replication and sharding database. Right now there are some bugs in the system, some improvements that need to be made, and definitely some default configurations that need to be changed before it's going to be a consistent database. It's definitely going for consistency instead of availability, but I don't think it can call itself consistent yet.
I don't think we disagree as much as you might think!
I'm trying not to critique MongoDB today, but rather thinking of what it will become. Today's implementation problems will hopefully be fixed.
I agree that without joins, MongoDB querying is very limited. I credit them enough to believe they will end up fixing that and supporting joins in some way. But then, they have "SQL with a different syntax".
And I definitely agree on the data-loss bugs. But again, presuming they fix those, they just end up with something very similar to a relational DB, in terms of the CAP theorem etc.
In short, even when/if they fix all the bugs and add all the missing features, I worry that we'll just end up back where we started.
Well, if that's truly the case, there's no reason you couldn't just translate SQL into JSON queries. (It's been a while since I looked at .NET mongo drivers, but LINQ in .NET basically already does this).
Much more interesting to me are systems that do something fundamentally different. e.g. explore different parts of the CAP trade-off, like Cassandra. Or systems that are built around the idea that data no longer lives exclusively on a server, like CouchDB.