Edit: Yes, I have been a bit lazy not to go and try Riak first hand. I think I will do it sooner then later but as of now, any light on the subject will be appreciated.
Technically, I found CouchDB more interesting as it's more "different" from other approaches.
anyway, mongodb and couchdb are more closely related to each other than they are to riak. a closer comparison to riak would be cassandra (also based off dynamo).
riak is based off the amazon dynamo paper and it is a pretty faithful adaptation at that. circular hashing, node chit chat protocol, eventual consistency. CAP theorem. uses n,w,r paradigm where n=number of replicas, w=number of replicas that must be available in cluster to write and r=number of replicas that must be available in cluster to read. so in this mechanism the balance of CAP is up to you, the programmer. disconnected writes written to two separate nodes due to a network flap are both returned to the client for conflict resolution. interestingly riak is written in erlang and uses rest heavily. because they do they were able to demo access libraries in virtually every language.
regarding mongo v couch:
mongo (written in c++) has sharding/replication and rebalancing (via addition to cluster) in production now. rebalancing via removal is on the drawing board. mongo also supports dynamic querying. mongo also supports bson (binary json). i also believe there is a 4MB max row limitation. multiple daemons for different functions. limited creature comforts like web gui, efforts underway to rectify.
couch is more complete in its restful implementation and general overall approach. i feel it is an easier setup and administration. one daemon pretty much handles everything i believe. couch is a never-delete database in that it always appends writes and internally versions them. this allows for very interesting replication topologies like master-master-master due to their unique versioning scheme. this will also lead to and "offline" couch version that you can run on your desktop. couch also has "fouton" which is their admin web gui.
the teams behind each of them are committed and talented.
That said, Bryan gave a fantastic presentation and really knows his stuff.