For some other helpful intros to Cassandra data modeling, check out [0], [1], and [2]. I've been working on trying to automate the process of coming up with a good data model for systems like Cassandra. You can see some pretty early results here[3] although there's been a lot of progress since then which will hopefully be shareable soon.
[0] http://www.datastax.com/dev/blog/basic-rules-of-cassandra-da...
[1] http://www.slideshare.net/nkorla1share/cass-summit-3
[2] www.ebaytechblog.com/2012/07/16/cassandra-data-modeling-best-practices-part-1/
[3] https://dl.acm.org/citation.cfm?id=2602624