With the old discussion system, changing the threshold/mode doesn't save unless you click the "Save" checkbox first. Maybe a "Save" command-button could be added to D2, replacing the implicit save with an explicit action. That's AJAXy. Or at least give me a preferences screen checkbox to not automatically save changes to my D2 thresholds.
I usually refrain from posting "Me too!" replies, but I also like to start small (high thresholds) and expand as necessary. I would prefer some kind of explicit "Save" button for the Full/Abbreviated/Hidden tresholds.
I often lower the thresholds when an article has only a few comments. But when I switch to the next article that has hundreds of comments, it is rather annoying to see all of them popping up and slowing down my browser until I can grab the bar and reset the thresholds (and then wait for the browser to catch up). This problem wouldn't exist I could load each new article with my default (high) thresholds.
It would also be nice to have a way to easily change the sort order to "Flat, Newest First, Ignore Threads", for moderating. I currently have to manually switch back to D1 when I get mod points.
Another "Me too!". Moderation became more difficult with D2. It would be nice to have a quick way to switch to "moderator mode" and then back to the user-defined D2 preferences. This concept could be extended further by allowing each user to define sets of viewing preferences and easily switch between them, but maybe that's a bit too complex. I would already be happy with a simple, easily accessible toggle between "moderator mode" and my usual preferences.