I think the "revolutionary" isn't so much a label for the technology, but the fact that people are "bucking the trend" socially. The RDBMS has been the "gold standard" for the last 30 years. But the RDBMS market has basically stopped innovating. Trust me, I LOVE postgres, but it's lacking a lot of useful features. such as:
- Ability to just "add nodes" to help performance, without massive administration overhead. This is non-trivial: You need merkel trees, vector clocks, gossip protocols, etc. See Cassandra and Riak (and the Dynamo paper).
- Ability to answer queries quickly - The RDBMS has tons of overhead, such as query parsing, index selection, query optimization, etc. Some of the Key-Value stores can answer a query before an RDBMS is finished figuring out what the query is.
- Ability to forget about the database enforcing schema -- that's the application's domain. (In the old days, the database had to enforce constraints because it was the "integration point". Now every app has an API, so the database is only used by one app.)
- Ability to forget about writing to disk. Writing to disk means you have to run disk-based recovery (fsck, special recovery code, etc.) It's slow and especially useless in the cloud (on EC2, you can instantly loose your disk and you don't get it back). Instead, if you want HA, you should do replication with fail-over (in multiple data centers.)
- Ability to forget about consistency - It's *much* easier to achieve HA if you drop consistency. Consistency isn't always a business requirement. (See Amazon, see airline overbooking policies)