Translation:You can't judge a book by its cover.
One is a statement of fact, the other is an instruction. "Don't eat my cookie" isn't something I'd say unless you can, and I don't want you to (because I want to eat it, because it's poisoned, who knows?) "You can't eat my cookie" is totally different (maybe I already ate it, maybe it travelled to another dimension, who knows?)
Yup, frequently useful, and people will slow down. I found the French in general and Parisians in particular quite friendly and helpful with my halting French, contrary to what I had been told to expect. Waiters, shop assistants, people on the street, everyone seemed most genial. I could hardly take a map out of my bag without someone coming up and asking if they could help. (No, I'm not a pretty young thing <g>)
True but couverture is feminine, that's why. It's hard to explain in English because English doesn't have gender associated to words, but think of it this way, if you wanted to say not to judge a book by its title (titre, masc.), then we would've used son titre instead of sa couverture.
C'est bon ?
the english idiom that is the equivalent of this is as dapetras says is "Don't judge a book by its cover." I don't think we should be marked as incorrect for using the normal english phrase. I think the explanation as to why you can't do this is pedantic and another example of imposing french structure on normal English: something that drives me wild!