Be able to detach a viewport to it's own floating window. Optionally maximize that viewport. This would allow you to use a second monitor just for a single viewport, this would be most awesome.

Viewport DirectX shaders are limited and buggy in many areas. They don't work in Nitrous, shadows can be very flimsy, compile times can be high, scene effect loader has not been updated for years (!!), SSAO does not play nice with transparency, transparency sorting

Please add another rotation mode to allow rotating around the mouse cursor (the point on the surface under the cursor), like you can in Cinema4d, 3dCoat and others.

This would be a fourth rotation mode and an addition to the current 'Orbit', 'Orbit selected' and 'Orbit SubObject'.

It's simply a MUCH more intuitive way of working.

Add support for Direct3D 10 & 11 for materials, material editor, and viewport.

Qualified normals, introduced with Max 2011 HotFix 4 are a great idea. Unfortunately they are buggy and do not always work. On top of that, it requires users to manually edit an INI file.
Making the feature more robust + allowing easy UI access to toggle is an essential step to getting making this feature more usable.

Guidelines to assist in modeling, something like Sketchup guideline.

Trying to play back a plate in the viewport-background can be a pain. Bad performance, no real 2d zooming, the plate isn't stored in the RAM and lots of other limitations. Maya's imageplane handles the job really good and I think max could use something similar.

Please make the zoom "adaptive" so it will slow down when zooming in on smaller objects, slow down when you're getting close to the object and speed up when far away.

Ability to utilize multiple video cards instead of one.

Maya shows HDR images on Viewport in reeltime! Why not Max? Its really cool for lightning...

A volumetric shader that can get data from objects, particles, atmospherics, Krakatoa and FumeFX caches, etc, and display it in the Nitrous viewport.

Change the viewport animation preview playback to work in ram player and enable ram player to scrub through each frame forward and back. This is critical when diagnosing what is going on in an animation and max has no way of doing this.

Right now the default playback option media player is horrible at this, you can attempt to output a quicktime friendly version but quicktime is only marginally better at scrubbing and max won't use quicktime only media player.

XSI does some of it, lightwave does some of it.
Let s have it all in the nitrous viewport:
*realtime depth of field
* motion blur (that could be used with the DOF, not one or the other)
* FOG, it use to work with maxtreme drivers, but it s gone now.
* volumetric lights

Get independent isolations depending the vewport, like in Softimage. You can see in one vewport only the object you are working on, while still see the whole scene on the others (great for contacts and interaction)

Like in Maya where you can have one viewport window show things like shapes and another not show shapes. makes it easier to have rig controls visible in one window and not another.

Some sort of pseudocolour overlay feature, which could analyse the current scene, and quickly display areas of high polygon concentration, object distribution and material types etc within the current viewport. Perhaps with similar visual results to the Mental Ray visual diagnostic options.

Transparent materials in Nitrous refine over time to reduce the "noise look" depending on your hardware. Currently in Nitrous, better hardware equals better looking transparencies. That's unacceptable to users with older hardware. This idea will allow the user to toggle between nitrous transparency and pre-nitrous transparency.