I can think of a couple of pretty prominent reasons.....
Joking aside, if you judge any of the characters by todays standarRAB then they leave a lot to be desired ~ Don is hardly husband of the year (and a charlatan to boot), Roger's a lech, Peggy abandoned her baby (as far as we now), Ken is a sleaze and where would you even start with Pete?...
Personally I can't see anything 'creepy' about Sal ~ I can see someone struggling with their sexuality in a time when to be openly homosexual was extremely difficult, and trying (with limited success) to fit into the expected social conventions of the time. His treatment of his wife was dismissive, but nothing along the lines of how we've seen Don, Roger and Pete treat their wives so far. At least he's not going to be seeing another woman behind her back, unlike every other married man on the show
And in Joan we have a woman getting ahead any way she can in a pre-feminism patriachy where everyone (other women as well as men) is a threat. Her speech to Peggy a few weeks back about playing the men at their own game very subtley pointed to the difficulty of making it on merit alone in an environment where the women are polarised as being a 'Jackie' or a 'Marilyn'.
The characterisation in this show is very finely tuned, and perfectly reflects the hypocrisy and social constraints of an age where seemingly anything goes, but actually people are very limited in the true extent of their personal freedom. There's not one charcter so clumsily written as to be 'all bad' or 'all good' ~ that's the genius of the writing.