Essentially you lose the warranty. On top of it if any app that checks for Knox status before functioning they may change or limit their behavior. If you don't use samsung apps then you have nothing to worry about.
as for the warranty, once knox is tripped there is no way to fix it. So basically samsung can refuse for any kind of repair and you are forced to pay out of pocket.(advice: get the most expensive protective case you can get your hands on).
It may bring the face value of your phone down if you plan on selling it.