Needing support for text indexes on embedded properties, with language not being enforced at the document level but at the properties itself, like this:

{
  name : "Mitch Pirtle",
  location : [
  {
    "language" : "English",
    "country" : "Italy",
    "city" : "Turin",
    "profile" : "I write code and play bass guitar."
  },
  {
    "language" : "Italian",
    "country" : "Italia",
    "city" : "Torino",
    "profile" : "Scrivo il codice e il basso suonare la chitarra."
  }
}

We already have many apps running like this, as we built a plugin for lithium that uses this approach; and the idea was lifted from a rails plugin that works the same way. I think this is going to become an increasing source of frustration as document schema encourages embedding different languages within the same document.