I don't want to just be able to see how large the elements on a layer are (or more specifically, how far outside the document). I want to actually see all of the elements that are not actually part of my image. Possibly be able to drag photos and layers to the outside of the document while I compose them into my final artwork.

If you've never seen someone use a 3d app, find a friend who has and let them show off their favourite workflow/object interaction features to you. If you've ever used illustrator you'll see a ton of things you wish illustrator had. Especially when you see nurbs curves.

[Easier blend tweaking: Need the ability to modify a blend in the contextual control bar when a blend is selected. For example, the ability to change the steps in a blend there...]

Many programs solve this issue by having “construction history” and/or providing access to the “node” that creates the effect. Allowing you to edit values on the fly in a side-bar. (See Gaita's Freehand Start Point example)

[...rather than have to go to Object > Blend > Blend options… every time you want to make a change.]
Maya addresses this by having the ability to “tear off” any menu panel into a temporary window. If you know you're going to tweak, undo, tweak, undo blends for the next 3 minutes then you could tear off the Blend menu before you start for easy access.

[Sub-select by Command-clicking through a stack of items.]
Maya has no less than 3 different ways of looking at all the objecs in your file. Depending on what you're looking for and why, one method might be better than the next. The current layers panel is cramped and cumbersome but they'd piss off to many people if they changed it. They should have a look at maya's Outliner, Dependency Graph, and Hypershade. Also look at similar things in other 3d apps for some ideas. (Graphic Styles are a lot like shaders.)

[Align to Point: The ability to align a whole object to another object’s selected point. If this is currently possible, I don’t know how.]
The way snapping to grids, paths, edges, and points works in Maya is a thing of beauty.
And other 3d apps have wonderful alignment, distrobution, and duplicate/distribute/align tools.

The #1 thing that I feel ALL 2d vector apps should learn from 3d apps in general is persistent object coordinates and pivot points relative to word (page) coordinates. In a 3d app, when I move the pivot point it stays where I put it. If I rotate an object it remembers that it used to be at 0deg and now it's at 87deg and at any time I can go back and change 87 to 0 and have it nice and straight again.

The other thing that need improvement is the scale tool. For YEARS myself and 4 other designers I've worked with have struggled with that damn tool. The simple scale widget of any 3d app is more user friendly and IMO gives better visual feedback when being used.

Other 2d to 3d similarities
“Expand” is similar to “delete construction history”
Compound Shapes are similar to having a single polygon object with 2 or more distinct “shells”
Gradient Mesh is similar to Vertex Colors
Envelopes are like Lattices
Bezier Paths are like Nurbs Curves

Any layer whose bounding box extends outside the document area should be investigated further

In Photoshop, select the Pen tool and then click on the drop-down menu at the far right of all the shape options. Select “Rubber Band” — you'll see exactly what it does.

Glad the other tips helped!