DiscussionTools changes h2's from serif-24px, to sansserif-bold-24px.
The change of font is probably enough to distinguish talk pages from article pages, which was the motivation behind this change.
As talk page headers (topic subjects) are typically longer than article headers, it may be helpful to reduce the font-weight and/or size to fit more text in, especially on mobile:
Original (24px bold) | 24px + normal | 20px + bold | 20px + normal | |
Mobile | ||||
Expanded | ||||
Desktop | ||||
Note that the existing MobileFrontend talk page experience uses and even smaller font size (16px, same as body text) for the heading also with no bold, although the heading is never shown next to text (it becomes a dialog header)