Should this support something like $slice as well?

$slice ounds like a good idea. Going after a single element is a special case of $slice, so it seems like we should just implement that, and support functional use ($slice) as well as array element syntax to call the same thing.

This would be very helpful in combination with $unwind.

I think it's important to note that for $match this is not a new feature, rather it appears to be a bug in a currently existing feature (and arguably for $project as well). See the documented syntax at http://www.mongodb.org/display/DOCS/Dot+Notation+%28Reaching+into+Objects%29#DotNotation%28ReachingintoObjects%29-ArrayElementbyPosition and the language for $match which says "The syntax passed to the $match is identical to the query syntax." See https://groups.google.com/d/topic/mongodb-user/7o5frOLbPQ8/discussion for examples showing limitations of $match vs query in regard to this.

At present this makes it seemingly impossible to make use of the individual values in geospatial fields. I'd appreciate it if this was reclassified as a bug.

Note: $match does allow referencing positions of array, at least in the latest master:

> db.array.aggregate({$match:{"a.0":1}})
{ "_id" : ObjectId("5282869acefcd1a08068e472"), "a" : [  1,  2,  3 ] }
{ "_id" : ObjectId("5282869fcefcd1a08068e473"), "a" : [  1 ] }
Fetched 2 record(s) in 0ms

This allows matching only documents where a is array or a is non-array, but the same doesn't work in {{ $project {$cond} }} for example.