It's a brutal limitation if you're expecting MongoDB to be "MySQL, except with more awesome because it's new and shiny!", sure, but once you read the blog post,

It's a brutal limitation if you're expecting MongoDB to be "MySQL, except with more awesome because it's new and shiny!", sure, but once you read the blog post, it's quite apparent why - memory-mapped files are blisteringly fast, and raw speed at the cost of other features (like relational constraints, transactions, and single-server durability) is a part of the MongoDB setup. If those are acceptable sacrifices, then great, enjoy a wicked fast data store. If they're not, then it's not the tool for you, no matter how much the cool kids chat it up.