When Somebody Writes...
One of my peeves about Duolingo is that when somebody writes to you you don't get a notification! I mean, it's kinda hard to scroll through 700 notifications(real number) to see if someone wrote to you.
Go to your profile page and click "Stream". Then you will see ONLY the things that are relevant to you. http://www.duolingo.com/fourarmsfan13579
To understand how it works, keep in mind that there are 2 streams: the one at the home page contains EVERYTHING that any of your friends does, and the one on your profile page contains only things that YOU do, or are addressed to you.
Then, like Clara said, there's the issue with comments under comments. If somebody comments under something which is already on your stream, you have to scroll down to that part of your stream. That is definitely a problem.