Prof. Quirrell basically sealed Voldemort's ultimate defeat.

Ok, not really.lol. But I was just noticing. Rowling says after the trio fights the troll together, from "that moment on" they became friends. “There are some things you can’t share without ending up liking each other, and knocking out a 12 foot mountain troll is one of them". We also know that before this incident, Hermione isn't speaking to them, and Ron had just hurt her feelings terribly after Charms, prompting her to barricade herself in the bathroom. Well, it's possible that if Quirrell never lets the troll in, Hermione never resumes talking to them or becomes their friend, they eventually drift apart (since they really weren't close at all at this point), and then Harry and Ron go after the stone alone, they get caught in Devils snare, have no idea what it is (without Hermione) and die right there. Voldemort may not have been able to get the stone, but he'd probably have a much easier time draining life from Ginny in COS. BOOM! Voldemort wins, Harrys dead, the end. But...Quirrell had to let a troll in.

(Disclaimer: This is of course assuming all the other details of the story remain exactly in tact, simply without Hermione. Also, this isn't a serious post so refrain from thinking of it as that.lol)