I acknowledge that slippery slope argument has some validity, but what I haven't seen so far is a good criterion of where to apply it.

I feel confident being upset at people who draw swastikas where they expect Jews to see them, or burn crosses outside the houses of black people. Although one can make all the same arguments ("if we embolden the blacks and Jews by giving in now, then they'll start demanding more and more rights until we have to believe as they do") I still think doing either of those actions is wrong.

Drawing Mohammed seems designed to harass Muslims in the same way that drawing swastikas seems designed to harass Jews. So where is the critical difference that makes one necessary and the other abhorrent?