In addition to the $or operator (http://jira.mongodb.org/browse/SERVER-205), I feel there is a need for a $and operator.
A possible way to write it could be:

$and: [

,

]

Why it is needed:
Condition blocks are physically objects. Thus, we cannot have the same property used twice, preventing the use of the same construct/property multiple times in a query.

Example:
Suppose I have a collection of user sessions.
Each session can contain an arbitrary list of key-value pairs, possibly repeated (ie. an identical key can be listed several times in different objects of the list). Additionally, each key value pair has a timestamp, for history purpose.

A document sample would be
{
session_id: 1,
data: [

,

,

]
}

Now, when I want to query for a session with an user named john, I would need to use $elemMatch, since I can't match the full element because of the variable timestamp, like below:

find({
data: {$elemMatch: {name: 'john'}}
});

Now, I want to look for john doe. But since I can't repeat the same $elemMatch key in the data condition object, I am stuck.
The query below would run, but ignore the first condition, since it is being overwritten by the second.

find({
data: {
$elemMatch:

,
$elemMatch:

}
});

Possible solution with $and operator:

find({
data: {
$and: [
{$elemMatch: {name: 'john'}},
{$elemMatch: {lastname: 'doe'}}
]
}
});