Making the characters have different strengths and weaknesses would've been too much of a clone of Resident Evil Outbreak, which itself was a non competitive coop RE game.
Putting such a system in Left 4 Dead 3 would've created massive rage. Especially with Zoey in particular. If they gave her too many strengths and not enough weaknesses, people would bitch. If they made her weak, people would bitch and that's just with one of the survivors, nevermind eight.
It would've created an entirely new tier system with the survivors and if they added puzzle solving to the maps, once again that's RE and I could see the Left 4 Dead community raging if rushing/speedrunning weren't easy or the game wasn't 'the Left4Dead everybody remembers' so purists would criticize it. Anything new would be passed off as not Left4Dead.
That said, if they kept versus, these sort of things might've helped the specials in a casual PvP mode.
