Please review the patch for Drupal core

may I ask, why core if it's performing so well as module?

Because the current subject field behavior leads to unwanted visitor behavior and/or useless comment subjects:

"Technically speaking the subject of most comments will be the post itself. So the comment subject line often ends up being something like, "Agreed" or "Another thought", which doesn't really mean much. However the comment subject line is used in the comment block and several other places in Drupal where comments are listed. If it is not enabled, Drupal will use the first few words of the post as the subject line." (from a Lullabot article).

So in my view, and that of others, the current behavior of comment.module is flawed.

well, comment_subject didn't make it for D7,
nevertheless, I guess there will still be issues regarding comment subjects defaults in D7

so probably we might push for D8,
but in that case a reorganization/reroll should be prepared to persuade Dries to change his mind (http://drupal.org/node/138632#comment-991345)
I guess that having more flexible/configurable default subjects might help

for now I'm closing this issue, since it is the oldest active
please reopen if you feel it deserves (or open a new one once D7 has been criticized about this)

as an interesting note before re-attempting to get comment_subject into core
http://drupal.org/node/6465#comment-296766
dated March 14, 2004
it reads:

This (Re:xxxx) has been discussed for the last 2 or 3 years in the drupal community. At some point (2 or 3 versions ago) drupal even had the Re:xxx as a default for comments. However, there are good reasons, most of them to be found on the abandoned http://drop.org village why "Re:xxx" is no good. For example, see //this posting from 2001.

I still think these arguments are still valid, despite the fact that I'm (was) a usenet addict. So while one is free to use this patch, I doubt it will be hitting the core. Feel free to make a module with an optional "Re:xxx" option.