He's somewhat borderline, but the post season stuff has to be a big plus.
I've said before that the best postseason pitcher of all time is either Schilling or Mo Rivera, but even without the postseason dominance he's a shoo-in for the Hall. A lot of his counting stats were depressed by being on some really awful Phillies teams. But when you look at his peripherals, even in those terrible Philadelphia years, it's pretty easy to say that he was one of the best starters in the league for almost fifteen years.
He's a reprehensible piece of shit, but he's a Hall of Famer.