Having a collection with elements with a 'tags' array, where each element in the array is {k:<key>, v:<value>}, with an index {'tags.k': 1, 'tags.v': 1}

A query of the form
db.coll.find({tags : { "$elemMatch" : { k : "somekey", v : "somevalue" } } })

correctly makes use of the index. However, a query like:
db.coll.find({tags : {$all: [ { "$elemMatch" : { k : "somekey", v : "somevalue" } } ] } })

does not make use of the index and produces a full scan
(note: the use of $all is to allow search matching on more than one tag)