...
new format for interface animations
improved render sorting and performance
better effects rendering
shader support
new input code
new pilot file code
dynamic ship/weapon limits
compressed VPs
better support for widescreen resolutions
better LODing support (one model could use multiple LODs if it's big)
proper frustum culling
...
And that's just the stuff that I'm working on.

There is a lot more too, but many things will actually show up before 3.7 (like normal maps, new FS2NetD code, improved memory management and texture handling, new lab functionality, new texture replacement capabilities, etc.).