Discussion subscriptions make for an easy way to stay on top of discussions you're interested in (although they've been broken for a bit so you might not have even noticed them), but its become pretty clear that there are some missing subscription types that make it hard to stay updated about other kinds of content.
Because of that, subscriptions now work for discussions, notes, and conversations which means you can get notified about any updates without having to have someone mention you to get your attention. Wee!
For now discussions subscriptions are the only one that allows for unsubscribing because they could become pretty noisy. The assumption with the others is that if you post a note to your feed and someone replies, you probably want to know that (not unlike Tweet replies). Likewise if you create a conversation, or jump into an existing one, odds are you're interested in seeing how it develops over time.
Are these assumptions right? I'm not sure and I can see the case for adding unsubscribing across the board (I'm a notification-phobe myself so eventually I'll probably want that), but for now while we're still small and growing I think the volume of notifications will be fairly reasonable so I thought we'd wait before adding in bells and whistles (and cluttering up the place with even more footer buttons... they're getting tight for space already in places).
As always, new features = new bugs, so if you spot any weirdness with subscriptions please mention them here, or in our never ending Site Bugs discussion.
I for one am looking forward to being able to stay on top of all the awesomeness that goes on here. No more endless clicking to find things with updates, now the updates come to you :)
Addendum: Worth noting is that these subscriptions aren't retroactive, so they'll only be applied to new content. If you reply to an existing note/conversation you'll have a subscription get created, but they won't apply for any previous content that doesn't get updated. If I were to apply them retroactively everyone would have a lot of outdated notifications to deal with (which is already true since the discussion subscriptions were broken for a while).
Double Addendum: In case you thought you'd reply to random things to test notifications, it ignores posts by yourself because you've already seen them. You'll only be notified if someone else replies. Feel free to mention me to get me to reply to something if you want to check it out though :)
Effectively, we've made this place work more like Facebook notifications, yeah? Someone comments on your status or one you've participated in - notification. But on sourced, you can't really opt out of this (yet). The only thing you control for notifications are ones on Discussions (via Subscriptions).