I appreciate you taking the time to respond to our concerns. Couchbase Server continues to demonstrate great performance and scalability in clustered benchmarks. Now, we see MongoDB demonstrating solid performance in single-node benchmarks. It's not a surprise.
I think MongoDB is well-suited to single node deployments, but you can't estimate the performance of a cluster by benchmarking the performance a single node. After all, it doesn't account for replication or the performance trade-off between availability and consistency caused by the master/slave architecture of MongoDB.
If you were having difficulties with the latest client library, we would have been happy to help. Thumbtack benchmarked Couchbase Server 2.5 before the 2.x libraries were available. They were released with Couchbase Server 3.0, the release you benchmarked. It's not appropriate to benchmark a current release with a library that was intended for the previous release. At the very least, you should have performed the benchmark with the current 1.x release (1.4.9) rather than an outdated one (1.1.8).
We'll have to agree to disagree on CAS.
However, durability is not defined by "writing data to disk". It's a property - data will not be lost if a node fails. A distributed database is durable when replication is synchronous. For example, persistence on AWS is not enough for instance with ephemeral SSDs. If the instance is stopped, the data is lost. However, if the data is replicated... it's not.