He ruined them by being more interested in the effects than the story. Also he'd rather sell tons of terribly made merchandise and spend time re-releasing the same movies over and over. When it really came down to it all George Lucas has is Star Wars and he's milking it for all its worth. Now Disney is clearly going to do the same, because hey, its Disney, but in any future installments of films without just the sole input of the Bearded One maybe they can get some quality back in the films.
That's the thing I like the most, while Disney doesn't always put out gold at least there can be some fresh perspectives for Star Wars.