1. Good idea, was considering that anyway as a way to reduce my database load, as opposed to saving every 4 seconds (if you change the drawing) which is what I'm currently doing. (I'm a hardware engineer and this is my first webapp: I just found out writes to the database are really really slow. Will explore memcached optimizations too.)
2. The intent of the refresh button is for when you're sharing the drawing with others (I initially wanted realtime refresh, but got worried about performance). Hence I put the button under the share panel. But I guess you're right, you would need to refresh more often than you'd need to see who else you're sharing with or to add people for sharing...
2.1 This should be trivial. Will do :).
3. (Not sure if you saw the video or already know this.) The pane on the left is the library pane (or "documents dock") that shows all your saved drawings. The idea is the current drawing is just like any other drawing in the library, just that it's got the focus for now. Or is this idea non-intuitive?
4. The issue is how often would you want to draw angular vectors, compared to rectangles. As an "optimization" for the UI, I allow users to draw a rectangle with just 2 sides rather than all 4 sides (which also works), because I expect them to be very common.
Thanks so much for the feedback! And I'm elated that you'd actually use it!
EDIT
5. You're the third person that asked for this feature, so now it's on my near-term to-do list. I don't think it's hard; just to eat the cost of the bandwidth and storage (which currently is very low).
6. Hmmm... I personally hate revisiting the toolbar each time I have to enter text, so I don't (yet) agree. This is on my "wait for more users to ask for this feature" list :).