As for queries, it kind of depends on your model. Mango queries are pretty limited (no joins, no arbitrary filters), so it's not necessarily as easy as you think to write one that hosed performance. A client could of course write one that doesn't use an index, which may or may not be a concern.
An easy option if it is though is just don't expose the `_find` endpoint, which effectively limits your users to the map/reduce queries you've written (unless you give them admin they don't have the ability to create their own).