99.9% web sites are CRUD, which don't really need full features of RDBMS, which is both an overkill and a bottleneck for scalability.

NoSQL is ironic word in that both Hive and Pig are gonna introduce an ANSI SQL dialect for scalable data queries :)

The NoSQL movement is really a NoBlindJoinsViaRDBMS movement, but the latter is less catchy.