It is an idiom used to express,that something is almost perfect if not for a small defect. It applies that nothing is perfect and no matter how something seems so complete ,there is always some imperfection,even if almost unnoticeable.

可以說 ： imperfect, unperfect.

China-English: American Chinese not enough. haha~~~

美中不足 = everything is fine except for one small defect (idiom); the fly in the ointment http://www.mdbg.net/chindict/chindict.php?page=worddict&wdrst=0&wdqb=%E7%BE%8E%E4%B8%AD%E4%B8%8D%E8%B6%B3