we recently upgraded to mongo 2.6.10 one of our replica set, after some time the master became unresponsive - it means that all queries were gathering into queue and mongo was not doing anything with queries, and replica set became in fact broken, so we rolled back to 2.4.10

https://mms.mongodb.com/host/detail/4e44f4be14587819403c9268/8980bca521bb5812857cbdc30eb06774

there you can see the queue size of 20k +

please fix asap - it's very critical