I think the tracking system will make it a lot more likely for discussions to stay alive past the couple of days posts stay on friends pages. It will make it much easier for people to participate in discussions that they happen to find by surfing or links, even if the discussions are happening outside their immediate circle. The new system plugs the gaping hole in the notification system where if I comment on user_example's journal, I only get emails of replies directly to my comment, but if exampleuser replies to the person who replies to me, I don't get an email. Similarly, it makes it possible to get notifications of replies to posts in communities that I didn't post myself, and so on. Being able to manage my notification messages onsite, rather than through my email, is also a bonus in my view.
I honestly think it could transform the way LJ works. Before subscription, LJ was frankly pretty hopeless as a discussion medium. Lots of people, especially in the sort of circles I move in, are trying to use it as such anyway, and getting really frustrated because several aspects are so completely wrong for that. This change, I think, will remove a lot of the worst obstacles. Yes, it's still a kludge added to something that was intended for journalling (remember, LJ was created before blogging as we now know it became trendy). But this tool, combined with the serious efforts of lots of users to force LJ into something that can be used for extended discussions, will convert LJ from hopeless to just about usable for this purpose. Since I like discussions, and since I treat LJ very much as a blog myself, I'm delighted about this.
Some people are upset about this development, though. To me, it looks very much like the kind of feature that if you don't like it, you just don't have to use it. But I'm finding myself tempted to behave like the kind of obnoxious twat who spends hours posting identical comments to everyone who has complained in the comments to announcement posts (hey, something else that tracking might make easier!) telling them why they have misunderstood the situation. So rather than being obnoxious, I'm going to post here and explain why I think people who are upset are misguided.
For a start, this is something that has been "in the pipeline" for about as long as LJ has existed, so it's not one of the new bells and whistles that 6Apart decided to develop because they thought it would be k3wl or to keep up with rival sites (MySpace, Facebook etc). Lots of people hate change, and seem to be complaining as a knee-jerk reaction because this is novel. Actually, it's something that should have been there from the start; the problem is that it wasn't introduced in 2002, not that it has been introduced now.
Yes, the push-pin icon doesn't fit into everybody's journal style. Most of that is going to be fixed from LJ's end fairly shortly, and the rest is fixable with a bit of style hacking. Anyone whose style is not going to be fixed by LJ's patches, is someone who has customized their layout pretty heavily already, so by definition they know how to do what's needed or at least are capable of following instructions.
The main concern, though, is not neophobia or aesthetics, but privacy. It's not enough just to say "anything on the internet is public, duh"; while that's true, it is still worthwhile for a site like LJ to protect its users' privacy within reason. The point is that the tracking feature is not harming anyone's privacy whatsoever, whether or not you believe there is any point in being concerned about privacy online.
You can't track anything that you can't already see. Not posts that you don't have access to, not screened comments, nothing. I'm pretty impressed with how careful the developers have been to make sure this is rigorously true. Even people who have grasped this are upset because getting email notifications makes it easier to track something, which is sort of the point, really! There are some circumstances in which it's true that having aggregated, convenient data has more potential for abuse than the fact that each individual piece of information is available separately, though. I just don't think this is one of them. Note that it is not possible to track every comment by a particular user, or every post in a community where a user is active. You can automatically subscribe to events that relate to you directly, but for events that relate to someone else, you have to choose to subscribe to each one manually.
Many people are worried about "stalking". I think what we have here is a problem of definition. Unfortunately, there are actual stalkers out there. They obsessively follow every detail of their victims' lives and may even use this information to harm someone directly, as well as the inconvenience and unhappiness of the stalking itself. Stalkers in the strict sense are not going to need this feature; they probably already use tools such as RSS readers, notifications of page changes, and just plain old manual bookmarking and repeatedly refreshing pages they are interested in. Marginal increases in convenience are not going to make this criminal behaviour any worse.
However, I think a lot of people are using "stalking" in a colloquial sense, to mean people they don't care for, or strangers, simply taking an interest in them. Yes, the tracking system might marginally encourage such people. I just can't find any reason to see that level of attention as a problem. If an action is so harmless that obscurity can prevent it, it is too harmless to be worth worrying if there is a little bit less obscurity. It's a bit like the way people get really, really worked up about being "friended" by people they didn't know or even people they had quarrelled with. The word "friend" is part of the problem here, and I think the word "track" may also have bad emotional resonances, because it does have connotations of being obsessively followed or even hunted down.
In the end, after years of arguing, LJ caved in and made a cosmetic change to the friending system: if you ban someone from commenting in your journal, and they decide to friend-list you anyway, their username doesn't appear on your info page in the "Friend Of" list. Of course, the banned person can still read your public posts and indeed they still get the convenience of seeing your public posts on their friends page. So in fact, nothing has changed at all, but this tweak made a lot of people happier. It might be that LJ will eventually make a similar concession with the tracking system, by allowing people to remove the push-pin icon from their journals. At the moment they're refusing to do it, because it's security by obscurity and thus completely ineffective; if the icon is hidden, one can form the tracking URL manually, or view the journal using a layout which does show the icon. But it would be trivially easy to do (given a couple of hours to hunt through the relevant code, yay Open Source, I could do it myself), and might make people feel better.
I guess the conclusion is that there is a fair userbase who use LJ as a means of communicating with a few select friends. For them, any feature which increases the networking and community abilities of the site can be a threat. I don't think they're right to feel threatened, though. Anything which really needs to be private can be kept private by using LJ's very good friends locking system. Hoping to keep prying strangers from reading public information is futile. It doesn't matter that it's futile, though, because most of the time, such prying strangers will find absolutely nothing of interest.
So, conclusion: yay tracking. Boo whiners.