Sembra quasi ok, ma devi assegnare anche il category_id nel join esterno sinistro. Altrimenti ottieni tutti gli articoli in evidenza dell'articolo:
SELECT i.*, category.name, category_feat_item.item_id AS featured
FROM item AS i
INNER JOIN item_category ON i.id = item_category.item_id
INNER JOIN category ON category.id = item_category.category_id AND category.id =1
LEFT OUTER JOIN category_feat_item ON i.id = category_feat_item.item_id AND category_feat_item.category_id = 1
ORDER BY featured DESC
