There are a number of things you already do get notified about, over email. You get emails when there are new messages, when someone replies to your comments, or when you’re sent a group message. You can see all the things you receive an email about here.
You can also subscribe to individual post comments, or forum topics/replies.
You can also subscribe to new posts (trainings, interviews, etc.) by logging in and then going to the bottom of the Members Area and clicking the “Get Sparks” button (add your email address first!).
You’ll receive group messages about important things like upcoming group calls, which you can add to your Google Calendar on Call Event page.
The only thing you can’t really subscribe to is group or sitewide activity posts. For that you’ll have to check the group or the activity page to see what’s new.