Here are a few things which have probably been said a lot, but I'm going to say them for the sake of stressing their importance.
Post Production FX (Glow, Maya's regular DOF, Light Fog) in Mental Ray. Even though they're not physically accurate, they can easily look as good with MUCH MUCH shorter render times.
Post Production blurred reflections - Mental Ray's blurred reflections look good, but they take up way too much render time and look noisy unless you seriously up the samples.
Folders in hypershade - complex scenes can have hundreds of shaders in them, a little organization might be nice
Multiple shader glows - c'mon A|W, do you really think we always only want to use a glow one specific way in a scene?
Although using HDR image-based lighting in Mental Ray is fairly easy, it's always nice to have a built-in UI for it. Which reminds me, can we have Hypershade and Maya properly display and preview HDR images?
Maybe a built in "fill volume" feature with fluid effects and particles - where you select a closed surface and the inside of that surface is filled with fluid or regular particles.
A special "tides/surfs" feature with Fluid oceans/ponds on beaches.