But, I could see how this would be interesting. I suppose rather than 'following' you could just select a box, and opt for notifications for comments on that post. There are some old posts that I really liked that I would like to set a monitor on. Huh, I'm going to think on it a bit. I really want to resist feature creep, but it is an interesting idea. I'd like to know if anyone else would use this.
I think this would be a great feature for hubski at the current density of users. It's a feature that increases in attractiveness when the user base isn't too large (large trafficked posts don't really need activity alerts, because there is always activity, though not all the time), but facilitates increased participation of that base. I think it's worth considering.