Ability to Pipe the results from one Query to another one (all server side).

db.purchaseOrders.find(

{VenderName : "10gen"}

,

{ponumber:1}

).pipeTo().poItems.find(

{ponumber :"$pipe.ponumber"}

)

to really spice things up you could add support for mapReduce piping as well (and the ability to mix them in any combination for as many times wanted)

db.purchaseOrders.mapReduce(...).pipeTo().poItems.find(...).pipeTo().venders.mapReduce(...)

preferably not to have the .pipTo() method call necessary. like so
db.purchaseOrders.mapReduce(...).poItems.find(...).venders.mapReduce(...)