The new Table of Contents, combined with the lack of page structure brought by infinite scrolling, cause a loss of intuition about the relative length of discussions. Old talk pages give an spatial indication of the size of topics (using the number of sub-sections and the amount of scrolling in the scrollbar) which is now lost, making it hard to find the most lively topics - everything looks the same in the new ToC.
You could add some secondary notation to convey similar information. The most obvious is adding the number of replies to the right of the topic title at the ToC. There could also be some visual cues, giving more weight to longer threads - for example, adding color to topics with >5 comments, and bold typeface to topics with >20 comments or several indentation levels.