Agreed, this post doesn't really say anything or point out what the tradeoffs really are between durability, scalability and performance. Battery-Backed RAID has nothing to do with the scalability/durability tradeoff of a sharded system for instance.
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The tradeoff is giving up durability to performance. MongoDB operates under the assumption that durability is not critical and so will not write your data to disk. That allows it to perform at the speed of say memcached.
For example CouchDB, which is another NoSQL solution in the same space of document stores, has durable writes (in fact it is ACID). You'll see a lot of benchmarks out there comparing these two (or MongoDB with MySQL) with the emphasize on the speed of MongoDB, while it would be more fair to benchmark MongoDB against memcached.