I would like the ability to add annotations (comments) to particular time points in my workout. It would work by highlighting a segment of a workout and leaving a comment just on that particular portion.

This way I can leave notes for myself or my coach on how I was feeling or what I was doing during a particular moment of the training session. It would also provide a way for my coach to ask or tell me something about a particular moment in my workout. Example comments might include: "I started cramping", "Watch showed high heart rate, but the effort felt fine", "your pace looked low during this interval, were you feeling ok?","went above prescribed pace, I was feeling good and wanted to test myself".

Soundcloud has a similar concept, where you can comment on particular moments in a song.