How to list the fixes in the new software update packages when compared to the current version?

Use yum-changelog -you might need to install this:

yum changelog 2013-Sep httpd\*

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Or, if already installed, use rpm:

rpm -q --changelog httpd | less

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I found these changelogs often relatively meaningless, since they are maintained by the packager. Often you will find changes like this: 'Updated to apache version X.XX'. Then you will have to go to upstream's website and look for yourself.

As for RedHat, do not expect changes or even new features. These are implemented only when upgrading to the next minor/major version.
The updates are most often back ported to fix some security issues. Normally it is perfectly safe to install them, no questions asked.

I downloaded "httpd-2.2.15-28.el6.centos.x86_64.rpm" and "httpd-2.2.15-29.el6.centos.x86_64.rpm" from "http://rpm.pbone.net", extracted them and compared the folders using WinMerge.

Both have an identical file called "CHANGES" under "usr\share\doc\httpd-2.2.15", which list out the changes. It only lists changes when the version goes through a major change, for example, from "2.2.14" to "2.2.15".

So I guess there is no way for us to know what are the changes from "2.2.15-28" to "2.2.15-29" huh...

I didn't find any file called "CHANGES" when I downloaded the "mysql" rpms.