One things that bothers me in WordPress is that each plugin looks like it came from its own world. Plugin authors have to build up own HTML markup (also for simplest things like form fields etc.) and they often apply their own styles. Some plugins use lots of colors, others use different typography/sizes etc. and with each plugin you install the backend looks more and more like a bad collage of different forms, fonts and colors.
It would be great if users would have some kind of API to build up certain parts of backend in order to preserve common look-and-feel.
I guess what I miss the most is helpers when writing/extending custom posts with meta fields. I'd love to have helpers like:
text_field($label, $name, $hint); that would build the table rows and saving me some TRs and TDs. Feels like I have to write too much HTML instead of focusing on the functionality.