Today, I played Doom for the first time. No, of course I'd seen it before - I had the shareware version and later the full game borrowed from some friend or other - but I'd always spent my time going through with IDDQD on or on the easiest setting, I'd never actually played the game all the way through on Ultra-Violence before (which is considered by players the world over to be the correct difficulty setting, one below Nightmare which introduces special rules like respawning enemies).
I kept count of my time and my deaths as I streamed the game, and the results were:

Episode 1 in 1:08
Episode 2 in 1:58 with 11 deaths
Episode 3 in 2:06 with 20 deaths
Total time: 5 hours 12 minutes (or one Dream Theater song), with 31 deaths.
You'll notice there isn't a death count for episode 1... because there weren't any! Part of this might be due to just familiarity with the levels as I played the shareware version to death, but the difficulty curve seems to be as you'd expect - a couple of levels caused problems on episode 2, and then things were even more tense in episode 3.
Overall, Doom is still a much harder and more frightening game than I imagined - I had thought I'd breeze through the whole thing in a couple of hours, but I was very wrong. It stays creative throughout its three episodes, with the moon bases of the first giving way to corrupted hellish hallways in the second and then more open environments in the third, and continues to surprise you in each style.
It also makes things very difficult for you if you're not saving and reloading (I managed to resist the temptation entirely) - if you die on a level, you're put back to the start with just the pistol and fifty bullets, and are expected to re-arm yourself along the way. For the most part, the levels are designed to allow this, but it means you start off at a huge disadvantage and getting going again can be difficult - this was particularly true of the last couple of non-boss levels in The Shores of Hell and the second level of Inferno, where I just had to wake a lot of enemies up, run around madly and hope for the best.
Ammunition is as critical a resource as health in this game - I hadn't ever been in the position of running out of it before (due to playing on the easiest setting, where all ammo pickups are doubled) but when you've only got four bullets, a fork and a pencil sharpener left, the game becomes tense survival horror, just hoping you can sneak around or goad the enemies into infighting enough for you to slip past their diminished numbers. Episodes 2 and 3 notably have greater ammunition shortages, perhaps due to the relative lack of the Zombieman and Sergeant enemies that drop ammunition.
I did episode 1 in its entirety (secret level and all), episode 2 without the secret level although I found its entrance, and episode 3 with the secret level mostly as it allowed me to effectively skip Mt. Erebus, which was giving me huge amounts of grief. A lot of deaths were due to that level when I was perfecting the route from the invulnerability to the secret level entrance via the rocket launcher - it's a very clever use of a by-product of the rocket launcher's splash damage, and in a single moment invented the rocket jump.
After that, the penultimate level Limbo is surprisingly easy, with large spaces, plenty of room to move around, and abundant ammunition - but its difficulty comes from the large lava pits that you have to cross to reach the buttons and switches that open up the goal, and managing the supply of radiation suits that you require to get across as you're guessing which teleport leads where. It's an unfortunate bit of guesswork in some otherwise great level design.
In the end, I finished the third episode with 3% health remaining as I took down the Spider Mastermind with my last rocket. Source ports like Doomsday Engine definitely smooth over the rough edges, but in terms of level and enemy design, the game still holds up shockingly well today.
I kept count of my time and my deaths as I streamed the game, and the results were:

Episode 1 in 1:08
Episode 2 in 1:58 with 11 deaths
Episode 3 in 2:06 with 20 deaths
Total time: 5 hours 12 minutes (or one Dream Theater song), with 31 deaths.
You'll notice there isn't a death count for episode 1... because there weren't any! Part of this might be due to just familiarity with the levels as I played the shareware version to death, but the difficulty curve seems to be as you'd expect - a couple of levels caused problems on episode 2, and then things were even more tense in episode 3.
Overall, Doom is still a much harder and more frightening game than I imagined - I had thought I'd breeze through the whole thing in a couple of hours, but I was very wrong. It stays creative throughout its three episodes, with the moon bases of the first giving way to corrupted hellish hallways in the second and then more open environments in the third, and continues to surprise you in each style.
It also makes things very difficult for you if you're not saving and reloading (I managed to resist the temptation entirely) - if you die on a level, you're put back to the start with just the pistol and fifty bullets, and are expected to re-arm yourself along the way. For the most part, the levels are designed to allow this, but it means you start off at a huge disadvantage and getting going again can be difficult - this was particularly true of the last couple of non-boss levels in The Shores of Hell and the second level of Inferno, where I just had to wake a lot of enemies up, run around madly and hope for the best.
Ammunition is as critical a resource as health in this game - I hadn't ever been in the position of running out of it before (due to playing on the easiest setting, where all ammo pickups are doubled) but when you've only got four bullets, a fork and a pencil sharpener left, the game becomes tense survival horror, just hoping you can sneak around or goad the enemies into infighting enough for you to slip past their diminished numbers. Episodes 2 and 3 notably have greater ammunition shortages, perhaps due to the relative lack of the Zombieman and Sergeant enemies that drop ammunition.
I did episode 1 in its entirety (secret level and all), episode 2 without the secret level although I found its entrance, and episode 3 with the secret level mostly as it allowed me to effectively skip Mt. Erebus, which was giving me huge amounts of grief. A lot of deaths were due to that level when I was perfecting the route from the invulnerability to the secret level entrance via the rocket launcher - it's a very clever use of a by-product of the rocket launcher's splash damage, and in a single moment invented the rocket jump.
After that, the penultimate level Limbo is surprisingly easy, with large spaces, plenty of room to move around, and abundant ammunition - but its difficulty comes from the large lava pits that you have to cross to reach the buttons and switches that open up the goal, and managing the supply of radiation suits that you require to get across as you're guessing which teleport leads where. It's an unfortunate bit of guesswork in some otherwise great level design.
In the end, I finished the third episode with 3% health remaining as I took down the Spider Mastermind with my last rocket. Source ports like Doomsday Engine definitely smooth over the rough edges, but in terms of level and enemy design, the game still holds up shockingly well today.