View event bindings. I'm often debugging pages I've never seen before and the only way I've found to know what's bound to what is to grep the js for live/bind/click/etc. or replace the event library (usually jQuery) with one where I've inserted logging into the lowest-level event binding functionality. This sucks a lot.
Show me HTML source over time. If I use curl/Tamper Data I see what the server actually sent, if use View Source I see the source after JS has run, if I use Firebug I see the current DOM. I care about all of these. Give me one place to click through them with diffs. Include steps for the JS changes, and a copy of the JS stack at each change so I can see who did it.