The core functionalities providing the "attach media to post" and the "gallery" functionalities should be dropped due to several reasons. The most important would be that it has flaws when it comes to un-attached media that should be displayed in posts or pages... also the tedious "delete old, upload new and re-attach it to the post" work just to replace an image with a better version of the same-sized image is just wrong!
Plugins (see NextGen Gallery plugin, etc.) are allready capable of providing this functionality.
Hook the core up with two or three filters and actions and you'll see perfect replacements of the media handling that's currently integrated into the wordpress core itself.
Just my 2 cents...