Arbitrary Attributes in Blender
I wish I could add new, arbitrary attributes to things in Blender, then associate an IPO curve with them. That's something I really miss from Maya.
That, and the ability to plug in crazy shit from one channel to another. Like using the color value from a texture to drive the position of an object — I wish I could do random stuff like that in Blender without having to spend hours scripting an ad-hoc solution myself.
Why would I ever want to do that?
If you have to ask, your imagination needs a jump-start.