For us the big thing, during the current refactoring of <a href="http://www.funadvice.com" rel="nofollow">http://www.funadvice.com</a>, is eliminating joins.<p>In testing of the new up-coming platform, that was a huge, huge win for speed. And we're a Postgresql shop too.<p>MongoDB allowed us to:
* Have embedded documents (very large performance improvements)
* Have arrays and hashes as "columns"<p>We also use Redis in a few crucial places, because of its really good support for lists (queues), and sets, besides, just its blazingly raw speed.<p>Downsites? Yes. Many rails-style plugins don't work well. But an upside is that we're forced to write leaner code and not depend too much on those.<p>Another downside, MongoDB is super-fast, but is still a work in progress in some places, and the ORM we're using (mongo_mapper) is somewhat of a moving target right now.<p>But hey, thats what happens when you're on the bleeding edge.<p>MongoDB:
* build-in replication
* basic sharding
* embedded documents
* very very fast