Though it is not marked as required, Criterion a a part of Query object not being handled as optional, which makes some things awkward and inefficient.
For example to fetch a list of 10 last published Content now we have to use something like:

new Query(
    array(
        'criterion'   => new Criterion\LogicalNot( new Criterion\ContentId( array( 0 ) ) ),
        'offset'      => 0,
        'limit'       => 10,
        'sortClauses' => array(
            new SortClause\DatePublished(),
        )
    )
)

While it would be more efficient and natural to just have:

new Query(
    array(
        'criterion'   => null,// or omit this line altogether
        'offset'      => 0,
        'limit'       => 10,
        'sortClauses' => array(
            new SortClause\DatePublished(),
        )
    )
)

Both Legacy and Solr storage gateways need to be updated to handle this change.

We should also document in API Query class that $criterion property is optional and what it means if it is not provided.
Also, SPI SearchHandler::findContent() should document for implementers that null as criterion needs to be handled.