Good post. Nice overview of your experience.
1. Save-or-die spells. One memorable event was several players were late (for our weekly Saturday noon until the wee hours of the morning AD&D 2nd game) and two characters, both in high teens in terms of level, went out on their own to deal with an informant in another city. Teleport over there, meet with them. Oh look, it's a trap, poison in the food. No problems... until the characters rolled a 1 and a 2 for their saves. Dead. With no other party members even in the same city. I think we had to true resurect them, and they both lost their primary adventuring magic items.
2. Lower hit dice and rolled HPs even at first. Back then, HD was just the size of the die you rolled for HPs, there was no self-healing. But a magic user started with d4 in HPs. I have played a character with 1 maximum HP before, a bad roll on a character without a CON modifier. (Oh, and CON mod to HPs maxed out fairly low unless you were a fighter class/subclass.)
3. Adventures (modules) were often adapted from Convention runs, which were designed to to be hard and kill off characters so they could get scored and ranked.
1. Yeah, save or die was big. Poison, or stuff like Hold spells in combat which take you completely out and may let an enemy give you a coup de grace before anyone can help you.
2. We adopted max HP at first level pretty quickly, but yeah, lower HP in general, especially since you needed a 15 or better to get any Con bonus at all.
3. Is something a lot of folks don't realize. We didn't use a lot of modules, ourselves. Most of our DMs were more homebrew.
4. Less resources at low levels. A 1st level magic user had 1 spell a day. Period. No cantrips, they needed to use darts, a quarterstaff, dagger or crossbow for the rest.
5. In-combat healing was needed and expected.
6. No OA rules or anything like that to protect your squishy characters.
4. Yep, though once you got some treasure options like flaming oil were on the table and could let M-Us do a bit more. And throwing daggers could be useful depending on how tough your monsters were and how forgiving your DM's interpretation of missile fire around melee was.
5. But it wasn't very powerful. In AD&D after Cure Light Wounds at 1st you didn't get another cure spell until Cure Serious, a 4th level spell. Cure Moderate wasn't introduced until part way through 2nd edition, as I recall.
6. Depending on the edition, disengaging from the enemy DID usually provoke free attacks. Certainly in AD&D 1E. And the group I played with most also used battlemats for combat, which I was told was because of a DM they had previously had who consistently had monsters run up on the wizards even when the PCs tried to implement formation tactics and block for them. So minis helped clarify things.
And things that made it less deadly.
1. Static saves. Saves only depended on the target. We had a high-teen/epic level campaign going on and you made your saves 80%+ of the time.
2. Expectations of loads of magic items.
2. Definitely enhanced 1; I definitely played at tables which DIDN'T have loads of items. Possibly related to my groups using fewer modules and not basing treasure expectations on them.