pic related, blue armor in E1M2 without too much effort for the player to reach.
you kill two zombiemen to your right at the beginning of E1M3 instead of just one.
you don't face shotgunners until you reach the other side of the nukage when you approach the blue key.
of course, the lights out trigger activates in the blue key room, but there's two lamps for the player to guide himself to get out of there.
there's just no way for the player to miss the secret near the exit leading to E1M4, it opens and the player can hear it clearly
if the player goes towards the secret leading to E1M9 then there he will fight the Pinky for the first time in the game
the Rocket Launcher in E1M9 is present; the monster trap always spawns Imps and nothing else
the switch that triggers the Pinky trap only liberates two instead of the 10 or so demons that are programmed for UV
E1M9 will be the first place where the player fights the Spectre (if he reaches the secret level, of course), it is located in the blue room near the exit. He may fight another one if he falls to the nukage
The first secret in E1M4 is left intact in ITYTD (backpack + shellpack + medikit), there's a shotgun near the center of the starting area that the player can reach easily
pic related, TWO radsuits for the player to take, hinting that he may need them for a special area later in the level. Speaking of which, if the player reaches the secret where you take the switch to the Soulsphere, he won't find the shotgunner present in UV and there he will meet the Rocket Launcher for the first time, if he hasn't done it in E1M9, that is
In the labyrinth he will fight the Pinky for the first time ever, once again, if he never went to E1M9
E1M5, to the left there's the green armor for the player to take, as how it should be (Note to mappers, DO NOT put such kind of primordial items in secrets near the start of your maps, it's annoying and lame as shit)
The secret area that contains the automap, chaingun, ammo box and backpack is intact in ITYTD, if the player manages to find that secret, of course. The visor in the final, blinking room is there too
E1M6, there's only a shotgunner guarding the blue key, the rest of the items in both secret areas that the player can reach when falling to the nukage are left intact
E1M7 is empty as shit, not a single shotgunner outside of the building, however the partial invis. sphere is still present, hinting that the player may reach it by finding a secret door.
E1M8, only 4 pinkies at the start of the level that you can dispatch easily by using the barrels; the same two barons and that's it
If the player manages to reach the secret area that contains the RL + Partial Invis., then he will most likely fight the spectre for the first time ever in the game, without counting E1M9
in the "labyrinth" which opens with the blue key, you can clearly find the unmarked secret by checking the alignment of the armor bonuses, they're forming an arrow facing towards the texture (not sure if this is the same on all difficulties). The boxes with rockets and the backpack in the secret at the end of the nukage are left intact too
There's not a single Spectre in the game, without counting secrets, up until the final area in E1M6, and instead of the swarm of them that attack you, there's just a single one of em to dispatch