I'm not sure if this was already suggested, but I feel after doing some modeling with a 'real' 3D graphics program, that SL really needs a couple of additions.
1) Compound objects: The ability to intersect, subtract, and union 2+ objects together would make life a lot easier for some things. Subtraction of one object from another would allow you to cut a hole in another object for things such as a door/window. And unions may help rid of those unsightly seams (in otherwords, make the objects as one whole object)
2) Bezier objects: These would be useful for making curvy items (like tails, horns, etc) a lot easier and realistic by simply drawing 2D splines (like flash animation drawing and vector images) and having the rendering engine 'fill' in the holes with it's 3D geometry of the selected primitive shape.
These seem highly possible, and would also open up a whole new range of object creation!

