I'm just wondering, if you like a post do you think that you should be alerted when someone quotes it (just like the post creator)? I feel if you like a post it's because you agree with it, feel it's a nice argument or some other reason. If someone quotes it then they intend to disagree/agree with it and provide a reason why - if you like that post you probably want to hear this reply.
So if you get an alert that someone has replied to a thread? You still read the thread, you just get an alert that someone has replied to a post you liked. Kinda like on Facebook how you get notified when you like someone's status.
I wouldn't want to be alerted for a post I liked... Think of how many times Kier gets liked, and how often he is quoted; he's had things quoted from two weeks ago multiple times today, that would just be spam at that point.
True, you'd have to visit the thread, but would most people actually read the thread - or just read the post in question and move on? That's what I'd worry about - that it would turn out to be a thread-killer. Not that I'm lobbying against it - I'm just saying.