I wanted to stick to the whiteboarding/sketching basics - you get a pen and an eraser. Perhaps it makes sense to add a few more tools, but I'm wary of adding too much, because I want it to be useable on really small screens, without keyboard, and with touch input, as opposed to only mouse.

For a sophisticated HTML drawing app (albeit not collaborative one, and not very usable without a pointing device), take look at the awesome DeviantArt Muro app (http://muro.deviantart.com/).