(disclaimer: I haven't looked at Cassandra in 6 months)

Riak doesn't involve Java. It sounds trite, but that was a huge plus for our team. It has commit hooks. Search. Builtin JS VMs for mapreduce. More straightforward administration. Most operations are automatic or on-the-fly. Even though I think Cassandra+Hadoop is a technically superior product for managing absolutely huge gobs of multidimensional data (especially with its support for range queries), Riak's ease of use is pretty sweet.

Plus the team is superhumanly helpful about addressing bugs and gotchas. Everything's on Github, and contributing is pretty easy.