After playing for a good ~80 hours, it becomes painfully obvious to me just how nuetered this game is. When thinking about the original lighting, a lot of the other design choices labeled as "mediocre" just make a lot more sense. Here are some examples I found:
Bonfires. Why are there so many bonfires seemingly strewn about at random. Well, torches are extremely sparse. It appears the intention was to rush to the next bonfire while the clock was counting down and the game makes more sense this way. You weren'tsupposed to stumble around in the dark for hours. I also think this is why enemies despawn, should you run out of light the game is not an absolute hell with a metaphorical blindfold.
Aesthetics Look at Huntsmans copse. The sycthe-wielding barbarians? They look like pasty white PS2 monsters with poor textures. Under light? Real flesh, tons of details / reflections on the metal armor. Tons of monsters look way different (always better) under light. Same goes for areas. Ever take a torch through Sinner's Rise? goes from an eyesore blue to a cool underground area.
Level Design
Undead Crypt. It's alright for a dungeon. But take a step back and look at it should the game be the darkness as originally intended. You know those areas with seemingly random gravestones? You aren't supposed to sprint through it. It's a mini-labyrinth when you don't have a torch. Specter statues are kind of pointless seeing as the surprise is no longer there. The hall with the greatshield enemies? Without a torch, you wouldn't even know what you were up against before they would start attacking. The beginning hall with the three mages? A scramble to the end where you couldn't see what was attacking you. This area was supposed to be dark. Like, really dark. This is evidenced by the hollow escort with the torch, and the fact that the syan knights aggro from MUCH farther away when holding a torch. It was intended to be completely different.
As some people below said about Bastille - imagine this. You know the large empty room before the sentinels? You open the door, and walk in. You hear footsteps coming from everywhere, upstairs and downstairs, as you scramble about in an open area without protection. It's an element of surprise COMPLETELY missing from the game when it's a gray box where every enemy is easily visible.
Why is the gutter so short and piss easy? Because if you actually couldn't see jack, it would seem much longer and the enemies (silent, sneaky hollows) would actually be a threat.
Bosses This could explain why some bosses are easy, though I'll admit that's a bit of a stretch. you ever fought Lost Sinner with a shield? Piece of cake. Doing that in pitch black with no lock on? Fuck that. Fighting her without a shield and just a torch is considerably more challenging, though probably preferable.
The main reason I believe they just jacked up the brightness in the final game is because the change in direction was so sudden - you can't redesign areas like that 2-3 months before launch.