Cassandra is a great project, as it is Hadoop, MySQL, etc. The issue I am raising is that it is not so much which project is better on a feature basis, but the fact that Amazon is able to offer it as a service, in a scalable way that no other vendor is able to do (with the exception of Google and, on a good day, Microsoft). Most other "traditional" cloud vendors, such as Rackspace, do not have anything remotely comparable to this, EBS, SQS, RDS, etc.
I also found it interesting that the storage media is specified and it's SSDs. Solid state will be hugely disruptive for hosted services, I've been hoping for an instance-by-the-hour service backed by SSDs and I'll surmise from this announcement that it won't be long before that shows up on the EC2 menu. Gimme :)