What should be the considerations for choosing SQL/NoSQL?

Digg have some interesting articles on this question. Essentially, you're shifting the burden of processing to writes rather than reads, which may be desirable in highly scalable applications. Cassandra specifically is also highly available.

Simplistically, Cassandra is a distributed database with a BigTable data model running on a Dynamo like infrastructure. It is column-oriented and allows for the storage of relatively structured data. It has a fully decentralized model; every node is identical and there is no single point of failure. It's also extremely fault tolerant; data is replicated to multiple nodes and across data centers. Cassandra is also very elastic; read and write throughput increase linearly as new machines are added.

To me, you don't have any particular problem to solve. If you need ACIDity, use a database; if you don't, then it doesn't matter. At the end just build your app. And let me quote NoSQL: If Only It Was That Easy:

The real thing to point out is that if you are being held back from making something super awesome because you can’t choose a database, you are doing it wrong. If you know mysql, just used it. Optimize when you actually need to. Use it like a k/v store, use it like a rdbms, but for god sake, build your killer app! None of this will matter to most apps. Facebook still uses MySQL, a lot. Wikipedia uses MySQL, a lot. FriendFeed uses MySQL, a lot. NoSQL is a great tool, but it’s certainly not going to be your competitive edge, it’s not going to make your app hot, and most of all, your users won’t give a shit about any of this.

I liked Ian Eure's rule of thumb: “if you’re deploying memcache on top of your database, you’re inventing your own ad-hoc, difficult to maintain NoSQL system.”

http://www.rackspacecloud.com/blog/2010/02/25/should-you-switch-to-nosql-too/