One thing that drives me crazy about WordPress are the frequent version upgrades. (I'm glad, of course, that WP continues to evolve.) It can be a real pain to upgrade when you have a lot of customizations and plugins -- and after all, one of the great things about WP is that it is so customizable.
I'm not eager to upgrade some of the sites I've built with older versions of WP -- why fix it if it works?
However, there is the issue of site security, and this is the main motivation to upgrade to a later version. Holes in security are often addressed by the next version.
What I would like is to have the option to simply install a security patch, or a few core files that address the security issues, without upgrading the entire version of an installation.
It seems to me that this is in everyone's interest because many people don't upgrade their installations, and thus are vulnerable to security issues -- which can give WordPress a reputation of being insecure.
Maybe there's already a way to do this (and if so, please post the relevant info), but it certainly isn't obvious.