Here is a description of the differences I see:

- In the post-merge image, the underlining I use in the mode line is
broken, as is the underlining of the current article in the Gnus Summary
buffer, and the latter underlining touches the bottom of the characters
in the post-merge image, while there is a (IMO more pleasing) space in
the pre-merge display.

- The fringe arrow in the Summary buffer is redisplayed in the mode
line.

- The character size in the mode line is larger in the post-merge image
than in the pre-merge image (in both it is font family Helvetica).

- The appearance of the non-ascii characters I use for threading and
separation in the Summary buffer differs; in particular, the separator
forms a broken vertical line in the post-merge buffer, while it is a
continuous vertical line in the pre-merge buffer (except for the line
containing the Indic characters, which is also misaligned in both
images).

Steve Berman