You can now follow other users!
To set your notification preferences (or opt-out of being able to be followed), go to Preferences -> Notifications -> Follow. You can also optionally enable notifications when users you follow reply to a topic there (By default, only topic creation)
A feed of all activity from users you follow is available at Account Overview -> Follows -> Feed
You can follow users either from their profile card (clicking a profile picture), or their profile overview.