* If I want to ensure that my data is written to three machines, all my writes will stop working in MongoDB if one machine goes down. With Riak, it will start issuing the writes to another node in the cluster and rebalance when the missing node comes back online.
* If I'm running MongoDB in a sharded configuration, if one of the shards cannot be reached, all writes will stop. With Riak, any node will accept the writes and, once the network issues are resolved, move them to the appropriate node.
That said, conflict resolution is hard and there's no real way to get around it when you're using a distributed database like Riak. With Riak, as Chad says you get "increased development complexity for massively decreased deployment complexity." There's no silver bullet, it's important to look at the trade-offs of each option.
