After installing RHEL7, I installed a few other kernels (the debug kernel, and then updated versions via "yum update").
I found that after doing this, when I reboot, the default kernel presented by GRUB is always the one at the top of the list (which in my case, is the debug version of the original kernel, not the latest non-debug kernel that I would prefer.) If I select the one I want and reboot, it is not the default afterward.
It turns out that the reason for this problem is that Red Hat left out a key line from the /etc/default/grub file.
They (correctly) include the line "GRUB_DEFAULT=saved" to tell GRUB to use the kernel last saved, but they (incorrectly) left out the "GRUB_SAVEDEFAULT=true" line that tells GRUB to update the saved kernel with the last one booted.
This is something that should be fixed in the future. I can't possibly be the only person needing to make this change.