I'm just proposing some solutions which are a bit more "modern", it's your choice if you want to follow them.

Overall it looks fine.

I'd do this in the following way:

<?php if ($output !== NULL):
    foreach ($output as $key => $value): ?>
        <tr>
            <td class="json json_key">
                <?php ehco $key . ':'; ?>
            </td>
            <td class="json" id="<?php echo 'json_display_' . $key; ?>">
                <?php echo $value; ?>
            </td>
        </tr>
    <?php endforeach;
endif: ?>

Basically keeping the html out of php.

I think the event needs to be passed as a parameter to the anonymous function in order for the preventDefault() to work.

I'd do this in the following way:

$.each(json, function(key, value) {
    var row = $('<tr></tr>');
    var td1 = $('<td></td>').attr({ class: ["json", "json_key"].join(' ') }).text(key + ':').appendTo(row);
    var td2 = $('<td></td>').attr('class', 'json').attr({ 'id': 'json_display_' + key, 'class': 'json' }).text(value).appendTo(row);

    row.appendTo($('#json_display'));
});

Basically making use of all the jQuery functions which do the job for you, instead of writing HTML in JS strings and using arrays to hold your HTML elements.

Ship It!