3 "Existance" does not exist :-)
source | link

This is an old question, but it seems Wordpress has fixed this "missing feature": now, according to Wordpress Codex is possible to check for existanceexistence (or non-existanceexistence) of the meta key, like this

'meta_query' => array(
    array(
        'key' => 'featured_image',
        'compare' => 'EXISTS', //or "NOT EXISTS", for non-existance of this key
    )
)

This is available as of WP >= 3.5.

This is an old question, but it seems Wordpress has fixed this "missing feature": now, according to Wordpress Codex is possible to check for existance (or non-existance) of the meta key, like this

'meta_query' => array(
    array(
        'key' => 'featured_image',
        'compare' => 'EXISTS', //or "NOT EXISTS", for non-existance of this key
    )
)

This is available as of WP >= 3.5.

This is an old question, but it seems Wordpress has fixed this "missing feature": now, according to Wordpress Codex is possible to check for existence (or non-existence) of the meta key, like this

'meta_query' => array(
    array(
        'key' => 'featured_image',
        'compare' => 'EXISTS', //or "NOT EXISTS", for non-existance of this key
    )
)

This is available as of WP >= 3.5.

2 Just a little typo
source | link

This is an old question, but it seems Wordpress has fixed this "missing feature": now, according to Wordpress Codex is possible to check for existance (or non-existance) orof the meta key, like this

'meta_query' => array(
    array(
        'key' => 'featured_image',
        'compare' => 'EXISTS', //or "NOT EXISTS", for non-existance of this key
    )
)

This is available as of WP >= 3.5.

This is an old question, but it seems Wordpress has fixed this "missing feature": now, according to Wordpress Codex is possible to check for existance (or non-existance) or the meta key, like this

'meta_query' => array(
    array(
        'key' => 'featured_image',
        'compare' => 'EXISTS', //or "NOT EXISTS", for non-existance of this key
    )
)

This is available as of WP >= 3.5.

This is an old question, but it seems Wordpress has fixed this "missing feature": now, according to Wordpress Codex is possible to check for existance (or non-existance) of the meta key, like this

'meta_query' => array(
    array(
        'key' => 'featured_image',
        'compare' => 'EXISTS', //or "NOT EXISTS", for non-existance of this key
    )
)

This is available as of WP >= 3.5.

1
source | link

This is an old question, but it seems Wordpress has fixed this "missing feature": now, according to Wordpress Codex is possible to check for existance (or non-existance) or the meta key, like this

'meta_query' => array(
    array(
        'key' => 'featured_image',
        'compare' => 'EXISTS', //or "NOT EXISTS", for non-existance of this key
    )
)

This is available as of WP >= 3.5.