Absolute showstopper for anything more than keeping track of simple stats or storing comments. Denormalizing has limits. Invariably apps will come to the point where you need to do joins and that is when you start cursing your decision to go with NoSQL. My experience is that NoSQL (including Mongo) is not a replacement to traditional RDBMS, but if you use NoSQL complementary to an RDBMS, primarily for real time performance, works quite beautifully. That said, there may be quite a few simple web app use cases that do not need RDBMS at all.
it's true you almost always will find yourself needing to join cross collection/table... I believe the recent support to integrate with hadoop should help this: http://www.mongodb.org/display/DOCS/Hadoop+Quick+Start when your reason for needing joins is for reporting (often the case for say financial reporting)
Also, the postgres integration (linked/discussed) here on HN
Comments
Absolute showstopper for anything more than keeping track of simple stats or storing comments. Denormalizing has limits. Invariably apps will come to the point where you need to do joins and that is when you start cursing your decision to go with NoSQL. My experience is that NoSQL (including Mongo) is not a replacement to traditional RDBMS, but if you use NoSQL complementary to an RDBMS, primarily for real time performance, works quite beautifully. That said, there may be quite a few simple web app use cases that do not need RDBMS at all.
it's true you almost always will find yourself needing to join cross collection/table... I believe the recent support to integrate with hadoop should help this: http://www.mongodb.org/display/DOCS/Hadoop+Quick+Start when your reason for needing joins is for reporting (often the case for say financial reporting)
Also, the postgres integration (linked/discussed) here on HN