Headshots in doom 1, the game that had auto aiming and no ability to look up originally? Also, fully 3d might make it slower to load in case somebody actually wanted to use this for real. I played a webgl doom clone once that lagged like crazy
Doom/II/Heretic/Hexen were 2.5D. That makes it easier to run on a 386 with fixed-point math, BSPs, and efficient z-buffers without busting out single-precision quaternions and 4x4 transformation matrixes. :)
Interestingly, Rise of the Triad, based on a modified Wolfenstein engine, is semi-3D with elevation changing.
Auto-aiming in the vertical axis only, IIRC it shot randomly all over the enemy. Enemies only had so many angle sprites IIRC and weren't really 3D.
There's a much better treatment of the Doom engine in Michael Abrash's writings, esp. Graphics Programming Black Book Special Edition. Trivia: his assistant once mailed me a replacement CD-R of the CD I lost from the paperback edition. I lose everything. Hehe.
Do you remember the Quake 3 hitbox (it was only one for all models), especially after a jump pad? Quake Live fixed most of these issues but back than things were pretty bad. Still way ahead of everyone else but yeah... it wasn't perfect