As thimbleshack says, it's a bit of an old chestnut.  I've amassed about thirty--odd items still awaiting feedback, making it roughly one in ten of my sales.  It doesn't worry me - I just use it as an acknowledgement that they've received my item, the same as I leave feedback as an acknowledgement that I've received their money.  Apart from getting one a little bit closer to getting a different star - it's almost like junior school, it carries as much weicht as a compliment slip.