I believe adding a $pull modifier - the opposite of $push - would be a
powerful addition to the MongoDB features.

For example:

db.group.insert({_id:1,text:'user group 1',users:[1,2,3]});

// add another user - atomic
db.blogs.update({_id:1},{ $push :

} );

// remove a user - atomic
db.blogs.update({_id:1},{ $pull :

});