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.
Something comparable for Cassandra exists with Solandra (https://github.com/tjake/Lucandra/). A major difference is that you'll have to post your documents manually to the Solr Instance running with Solandra while Riaks post-receive hooks automatically do that for you.
Also: the setup is by far not as big of a pain with Riak
Local graph databases avoid the extra seeks by keeping extra copies of the destination vertex immediately next to the source vertex on disk, but I don't think Riak is doing this, or that it is actually practical in an eventually consistent system.