Instead of Dumbitch, the Defense Professor in Harry's Fifth Year is a man who believes that all conflicts can be resolved through good communications skills (well, conflicts between beings that can think, anyway - he's not so daft as to think you can talk a dragon out of eating you), and thus, he tries to teach the kids to communicate better, rather than teaching them to fight. This isn't so useful for the OWLs or NEWTs, so the DA still happens, but the students learning to communicate better does lead to other changes (like, say, Harry opening Sirius's present, and using the damn mirror). Another difference is that while he doesn't think Voldemort is back, his reaction to Harry insisting on it is to play amateur psychologist during detentions, and perhaps eventually becomes convinced that a would-be Dark Lord is trying to use Voldemort's infamy to his own advantage (which at least gets him to believe that there's a threat out there, even if he still thinks that talking will solve everything).
Was thinking that the teacher (who really needs a name) dies at the end of the year, due to trying to talk the person he thinks is a fake Voldemort out of starting the War up, again - or trying to talk Bellatrix out of attacking, because she's one of Voldemort's Dragons (WARNING: TV Tropes link). Whether he realizes that it's the real Voldemort or not doesn't really matter; if he isn't killed, he'll end up in a bed near the Longbottoms. Still, as misguided as his approach was, he at least managed to get Harry and his friends to realize that communicating better with your friends and allies (and potential allies that aren't currently enemies) is a good thing (even if communicating with your enemies appears to be useless), and instill in them the skills to do so. This can lead to some quite large changes in canon, no?
Was thinking that the teacher (who really needs a name) dies at the end of the year, due to trying to talk the person he thinks is a fake Voldemort out of starting the War up, again - or trying to talk Bellatrix out of attacking, because she's one of Voldemort's Dragons (WARNING: TV Tropes link). Whether he realizes that it's the real Voldemort or not doesn't really matter; if he isn't killed, he'll end up in a bed near the Longbottoms. Still, as misguided as his approach was, he at least managed to get Harry and his friends to realize that communicating better with your friends and allies (and potential allies that aren't currently enemies) is a good thing (even if communicating with your enemies appears to be useless), and instill in them the skills to do so. This can lead to some quite large changes in canon, no?