ADDiCT, you're completely wrong. That kind of aiming like in zDoom IS entirely possible in Blood. Mouse-aiming does work in Build games and it is a perfectly viable way of playing the game. Saying Build games don't have mouselook and that they were meant to be played exclusively with the keyboard is just silly.
The solution to ChisChis' dilemma is actually pretty simple and works wonderfully. It has nothing to do with resolution.
The problem lies within the Build setup program. The mouse sensitivity setting in the mouse setup screen only controls HORIZONTAL sensitivity. Not many people know this, and assume it also sets vertical sens. To actually control vertical sensitivity, which is what is turning people off mouselook in build games and making aiming unusable and impossible, you simply need to go to advanced mouse options > Mouse Y axis scale. Then just drop the slider down to about 0.60/0.70. I've found that a horizontal settings of around 0.60/0/70 is great too. I usually leave X-Axis scale at 1.00.
That's it, you have great mouse-aiming in Build games. I found this solution on the 3DRealms forums. The reason mouse-aiming is good in JonoF's port is because the sliders in the GUI launcher that control mouse sensitivity are actually linked to the X-Axis and Y-Axis scale controls found in the advanced mouse options in the Build DOS setup program. There's no setting for mouse sensitivity (only X-axis) and thus, no confusion.
A real, actual problem with mouse-aiming in Build games is the stickiness when trying to aim very slowly and accurately. The only Build game that doesn't have mouselook stickiness is Shadow Warrior. For the rest, it doesn't really affect normal gameplay, but it can be a bit annoying when you're striving for pin-point accuracy at an enemy far away. This problem doesn't exist in JonoF's Duke3D port though (it used to). I have no idea why Dosfreak wouldn't use mouse aiming with JFDuke3D, it's excellent
Also, Quake 2 wasn't the first game with mouse-aiming. Infact, Quake 1 wasn't the first game with mouselook either. The Terminator - Future Shock (1995), to the best of my knowledge, was the first game with freelook (up/down head-movement) mouse-aiming.
Whether a game is 2.5D or 3D has absolutely nothing to do with the authenticity of mouse-aiming. In video-game terms, Duke Nukem 3D has mouselook. In technical terms, it wasn't true mouselook like in a true 3D game, but it WORKED like mouselook within the confines of the 2.5D engine. It allows you to swing your head around. While moving your legs with the keyboard. That's the point.
EDIT: I'll see if I can upload a video captured from DOSBox or JonoF's port demonstrating smooth mouselook.