First of all:
flashlights supposed to shed light not cast shadows lol
I don't even.....
It's just the way how the 3D industry refers to this phenomenon. It's not the light casting the shadows but the objects and surfaces lit by the light ofc, but they still associate the "shadow casting property" with the light involved in the shadow calculation,. In real life everything works by the law of physics, but in a 3D engine, you have to calculate and rasterize everything, and programmers don't calculate all the shadows if you can't see some of them or if some are not needed for certain reasons (see bellow for more)
I don't care that much about AA, I definitely don't need 16x but it's a nice addition.
Idtech5 only supports 4xAF, and (iirc) forcing it to more even from the drivers won't do anything. Since the game is super-optimized now and runs fluid on just about anything (mostly thanks to the new idtech5 renderer and the GPU skinning), I think that any options which can increase the detail are welcome by the PC gamers with newer/stronger hardware. Probably it was very easy to add this modifications, so they did it to counter the fixed lower AF.
Why disable checkpoints? They won't hurt.
Doom3 is a horror-corridor shooter, which means that it can get repetitive very fast if the tension/adrenalin drops (same enemies on similar corridors, same task... etc), so it's only really fun if you play it on the hardest settings and if the game punishes you for every little mistake you make. The game just simply fails to deliver fun if you play it on easy settings imho. They already added more ammo and health into the BFG edition to make it playable for the console gamers and that combined with the checkpoints made the game super easy even on the Nightmare level, so an option to disable the checkpoints is nice.
Didn't notice any changes in default brightness, and what was wrong with it in the first place?
The BFG edition was too bright indeed, but I also did not notice any change, so it was probably a minor tweak.
They also said that they added shadow fx to the flashlight. What the hell can that mean? .............Anyway I didn't notice
any "shadows" at all with that option turned on.
..........................
I haven't noticed any projectiles' shadows
Shadows can be only seen on lit surfaces. idtech4 doesn't support ambient lighting or partial/soft shadows, so if a surface is inside a dark shadowed area, it will appears totally black, you won't see it without a light, - and because of that - if a shadow is casted onto a shadowed (black) area, you won't see anything about that either, since it's not possible to distinguish black from black.
The flashlight was moved to the left side on the BFG edition, and (probably because of the console versions) it did not cast any shadows to make it less demanding GPU wise. While it was possible to turn on the shadows with some console witchery, but even than, the shadows would have still originated from an imaginary lamp on the weapon side of your body (from the right, like in the original doom3), and not from the new location on your left, so they had to fix that too. It's casting shadows now indeed, it's just hard to notice sometimes because your own POV is almost the same.