Weiner wrung some good storylines out of Sal, and he was a likable character (well-played by Batt, who one hopes has made something of a name for himself). But this is actually pretty good news for those of us expecting more great things from the show.
As I've sort of said before, one of Mad Men's great attributes is its awareness of the way time passes, things change, and people drop in and out of lives. Rather than cling futilely to some prelapsarian illusion of continuity, Matthew Weiner has made the painful decision to jettison a popular character, which suggests that Mad Men will continue to be a show about nostalgia rather than a show pandering to it by giving in to the wishes viewers who don't want anything to change.
Still... Save Hildy!