Similar concept to SERVER-4079. Also related to the needs behind SERVER-458.

Allow a custom compare function for sorting.

db.people.find().sort((_foo_bar_baz: function(a, b) {return (a.foo + a.bar + a.baz) - (b.foo + b.bar + b.baz);}})

This might also make sense (in addition to the previous syntax, not instead).
db.people.find().sort(function(a, b)

{return (a.foo + a.bar + a.baz) - (b.foo + b.bar + b.baz);}

)

I keep wanting this. One case that keeps popping up is sorting tabular data. Often times columns are computed from the contents of a couple of fields and the best sort order in the mind of the user will not actually have a simple relationship to any of the stored fields. The current workaround is to store pre-computed fields where possible, and disable sorting where this is not possible (or not practical), but precomputed fields are often not a good option (especially when the sort is rare).