MongoDB has its problems (for instance document schemes can't be enforced, disk space consumption concept is unintuitive,...) and relational DBs have their strengths (ACID, SQL is standard among DBs,...). But very few people ever reach a point where the global write lock becomes a problem. 10gen knows exactly which features are vital to most of their users and which are not...
The relational model takes this observation and concludes that there should be no "natural hierarchy" in the logical model (the physical model is a separate question.) It's a _theoretically_ beautiful idea. The counter-intuitive _real-world_ result is that the theoretical approach also yielded faster systems than alternative philosophies. I think that's the reason why the relational model has dominated for the past 30+ years.
You might well argue that that's because a _good_ NoSQL implementation has yet to be created (the no true Scotsman argument). That may well prove to be true one day, but I would bet that a good non-relational implementation will be a derivative from the relational world (Postgres with hstore, or Google's F1 with Protobufs); not from a project whose starting axiom is to get rid of SQL/relational.
Personally, I would much rather perform a complex query in SQL than NoSQL. The base-line is likely to be not-terrible, which may be good enough for single-use queries. Optimization is much easier (IMHO) - SQL is a DSL for querying and indexing. Compared to having to write code to do that (which seems to be the primary NoSQL approach), the DSL approach seems much more efficient. SQL isn't perfect, but it beats no-DSL-support or another DSL that is even worse than SQL :-)