This feature would provide a really good way to keep track of works in progress, and would really help with organization. I know that organization can be a really big deal for any particularly long or complicated project. Ableton is also rich and complex enough that it can be very hard to collaborate via email/dropbox, and this feature would make that significantly easier.
What I propose is that right under the clip overview we have the option to show a second timeline: the NOTES Bar. This is Part 1 in the illustration below. It looks a lot like the clip overview. Additionally, you have another box at the bottom of your screen, lets call it the Notes Box. When you right-click (or control-click) on a clip, a selection, or a spot in your arrangement, your menu now has a "New Note" option. If you select that, the note shows up in the Notes Bar at the top of the screen, and you can edit that note in the Notes Box. Visual space and clutter is another big problem for Ableton, so you should be able to turn all of this on or off in the View menu.
As you can see from the illustration, there is a colored line indicating the length of each note (if the note was created on a lengthy selection, loop, or clip in the arrangement). As you move or play through the arrangement, whatever notes apply to the position you are at are displayed in the notes box. They should probably automatically number themselves in chronological order, I would think.
Anyway, I think this would be a really great tool. I know nothing about how complicated it would be for Ableton to produce, but I think that it would be very easy to use. It is also an expandable idea. Maybe you have the option to choose what color a given note is. Maybe you could make it possible for the user to tell Ableton to automatically set a note under certain conditions, say have your limiter print out a note whenever it hits beyond a certain point.
Pretty cool, huh?