When watching a thread (not by replying, by clicking the link) you're given an overlay asking if you want to receive e-mail notifications. This is an intermediary step that is already set on a global basis in our preferences and slows* down the watch thread function. If users wish to add e-mail notifications on a per-thread basis maybe an icon could be put there or, by clicking "watch thread", a second link appears next to it allowing you to enable e-mail responses. I agree that actions that could have adverse or irreversible effects should have an overlay (logging out/marking read) but ones with no real side-effects (liking/watching/following) shouldn't need one. I know you can just "OK" it (return key), but that's not the point!