You can add the following graphs to watch on the Couchbase console. These graphs can be de/selected by clicking on the Configure View link at the top of the Bucket Details on the Couchbase Web Console.
Disk write queues
The value should not keep growing; the actual numbers will depend on your application and deployment.
Ram ejections
There should be no sudden spikes.
Vbucket errors
An increasing value for vBucket errors is bad.
OOM errors per sec
This should be 0.
Temp OOM errors per sec
This should be 0.
Connections count
This should remain flat in a long running deployment.
Get hits per second
Get misses per second
This should be much lower than Get hits per second.