An improvement that I would like to see for WordPress is the ability to set a relative "horizontal width" like "full-width", "half-width" or "third-width" for photos and other images. Then when an image is loaded is displayed in WordPress, it is automatically and proportionally scaled to the width dependent on the client's display resolution.
Typically, I would have to call a particular display resolution for images that I show on my posts. This can work with most current-issue displays but as we move towards pixel-dense displays on tablet and desktop clients (think the 3rd-generation Apple iPad), this may not cut it anymore. Similarly, some of us may print posts on high-resolution printers and we may still want that detail in the hard-copy.
As well, I find that the WYSWYG editor doesn't do well in determining the way images and text look for my theme's "copy width". I have to end up saving and previewing my post so I can make sure that images and text fit properly, and is important if I want to have the copy "snake" amongst the pictures to create viewer interest.
[sig moderated as per the Forum Rules]