Since several incidents of page-widening caused by a long chain of child posts a hard limit on how many levels of reply would be indented, after which all further sub-posts are displayed at the same indentation e.g.
where the last 26 comments are all displayed at the same indentation, but consist of an entire tree of comments and replies under grimmjeeper's comment. This makes is incredibly difficult to follow exactly what's going on without - opening any of those 26 comments doesn't help, because it again limits the indentation, whereas under the old comment system it opens the comment at the root so you could see the sub-tree below.
In Chrome this max indent is only 310 pixels or so from the left edge of the browser, and given that according to w3schools.com's stats show 85% of users have a display resolution of greater than 1024x768
there's plenty of room to allow more indent levels before page-widening becomes an issue.The old comment system had 375 pixels before hitting the indent limit, under D2 you could add another 4 or 5 levels and get the same. Having it configurable would be even nicer :)