Calling Doom "2.5D" is an insult to the 3D feeling of that game. You mean that just because the Doom world doesn't have multiple floors on top of each other and no doors open sideways it's not 3D? Doom is 3D because it looks and feels 3D! Even the most realistic game's 3D is a two-dimensional illusion on your flat computer screen, how come Doom's illusion doesn't count just because its math is more limited?