While the oplog does provide some semblance of RethinkDB's changefeed, it's not nearly as powerful. With Rethink, you say you want a query, and rethink will let you know about changes to that query. Mongo just says "hey, here are operations that were done", and leaves the reconstruction to you".
So I guess Mongo+Meteor match up with RethinkDB... sort of.
It's also worth noting that changefeeds are highly scalable: you can run tens of thousands of them on a single node, and scale them out linearly from there (even as they're scoped to specific queries.)
Obviously the performance characteristics will be impacted by the volume of changes that arrive to the database, but the architecture to support this is highly parallelized (all the way down to cores on the CPU.)