in terms of understanding basics, i feel like a lot of the game play basics are taught to the player in a satisfying and cohesive way, like the wrench throw/force field power box being a grate one, that is honestly perfect, same with slide kick teaching, but with the vents there was never a moment in the first map that really taught the player that was a game play feature, for me in my mind i knew it as one due to half life, but for a new player i think it would be grate to just have a moment in the first map were the player must break and crawl through a vent to progress, that way its in there minds that this is something i can do, much like how you taught the players with the wrench throw and force field fuse box shut downs.
and same with the med kits and jump, the medkit is hidden away in a vent as a secret that can be missed by the player, when that is a important game play aspect for the whole game, so in my mind you could kill two birds with one stone, by having a part in E1M1 that forces you to break a vent and crawl through it to collect and use a medkit after being hurt, then the vent leads in to the room with the shotgun, and you can sneak up behind the enemy looking at the vending machine to dispatch him with ether the new gun or your wrench, this both allows the player to be introduced to the first real enemy of the game in a safe way, as well as know how to heal and how to attack with the wrench.
again this is purely my own lvl design brain, but hopefully its some food for thought.