Nice to see this kind of exploration, but like Malte, I feel those extra buttons are unnecessary and too complicated (even a second button). We give users too many options already and I did not understand those buttons until I read the alt text (even experimentation did not make it obvious). Expand collapse per tree/message is enough.
More importantly this introduces a dependency on javascript that does not gracefully break (we are trying to reach level A accessibility compliance for .LRN and this would have torpedoed that effort). Try it yourself: turn off javascript and 3rd level message bodies are not loaded (notice that Jeff's simple expand collapse works). I know it makes it much harder, but end user pages should be accessible.
