Cassandra offers a mix of commutative operations (sets, maps, increments), an eventlog model, and lightweight (paxos-based) transactions. Unlike a key/value database like Riak, Cassandra can update individual fields of a row or document independently, which simplifies things enormously.
>>jbelli+(OP)
Cassandra suffers from the same problem and can drop updates. The Paxos transactions were and maybe still are an absolute joke as exposed by Aphyr.