I suggest a feature that allows to set a value to a field only if it is not already defined.
This could be similar to $addToSet and might be called $init:

db.users.insert(

{uid: 15, name: "myname"}

)
db.users.update(

{uid: 15}

, { $init :

{ status : "offline" }

}
// the status field wasn't set so it is initialized

{uid: 15, name: "myname", status: "offline"}

db.users.update(

{uid: 15}

, { $set :

{ status : "online" }

}
db.users.update(

{uid: 15}

, { $init :

{ status : "offline" }

}
// no effect because the status field was already set.

{uid: 15, name: "myname", status: "online"}

Note that $not and $exists won't help in case you want to set some fields and init others:
db.users.update(

{uid: 15}

, { $init :

{status : "offline" }

, $set:

{age: 23}

}