Personally there were (3) major things that never made it into OB that I had hoped to see.
1) Extended Animations for Extended PC/NPC Skeletons. Skeletons to support flowing robes, wings, 4-arms, etc were developed by several modders but the time required by capable animators to update all 100+ PC/NPC animations did not support such a labor intensive project.
2) Dynamic Animation Switching. Ability to switch animations on-the-fly or on a per-item basis would have permitted new weapon types. No solution was ever presented to overcome this game engine limiation.
3) Dynamic Lighting on Statics. A limited solution was developed as proof-of-concept but full implementation was too daunting as it had to be applied on a per mesh basis and even then had limitions. Again this was to overcome a game engine limitation.