Why I don't get notifications when someone "marks up/down" or "votes up/down" a comment of mine is beyond me.
Also, why send notifications by mail? Why not just pop it up when I log into Duolingo? That's not too much to ask. At the moment, if I have to see what comment was added to a conversation I'm following I have to go and check it in my mail.
Why Duolingo why?
Well, I find "notify me" versus "e-mail me" (in the settings) pretty clear. You could put it in a tabular format, but please keep it simple.
Also, could e-mail notifications be in plain text? My e-mail client cannot display html e-mails; especially with the large amount of html code, which is sent with the notifications, it is very difficult to find the content of the message.