The next item I'd like to cover is experience.
The experience point system is something I think had a greater effect on the play style of older editions than is immediately obvious. Not only did it emphasize a different type of adventuring than newer editions did, it also had a built in balancing effect.
In older editions, experience points were awarded primarily for the successful acquisition of treasure and, to a much lesser extent, defeating monsters. For example, a group of 6 orcs might be in possession of a gem worth 500gp. In terms of experience points, this would, altogether, net the party about 590xp; 500 for the gem and 15 or so apiece for the orcs.
That's a huge difference there, the gem is obviously worth a whole lot more to the party than killing the orcs would be. Effectively, it changes the whole goal of the encounter - simply fighting with and defeating the orcs is not the point, getting that expensive gem is the point, the gem is what allows the party to increase in power, and gives them funds to spend on carousing and training. Killing the orcs may simply not be worth the risk of getting killed.
Realizing this suddenly opens up a world of new options to the players - getting that gem by guile, stealth, or cunning may be safer and even easier than going toe-to-toe with the orcs.
This sort of experience system became another baseline of older edition games - this was a game of exploration and player skill as much as, if not more than, it was about kicking in doors and killing orcs. And yet again, future editions would erode this core concept.
Beginning with 3E, a new experience point system was introduced. It was mathematically coherent, followed a progression, a formula, and would largely turn the game into Fantasy Fight Club. Experience from this point on was awarded almost exclusively for killing monsters. Now, the encounter with the six orcs holding a 500gp gem was worth 900xp, 900 for the orcs, and butkis for the gem. Getting the gem through guile, stealth, or cunning now had absolutely no effect on advancing your character's abilities - killing orcs is what makes you level up.
There is a "story" xp award for 3e, by the way, but it was largely small and arbitrary, and did little that I could tell to emphasize exploration (or even story) over combat.
Experience also had an effect on how characters progressed. In older editions, the experience total needed to advance a level basically doubled at each level - for example a class might need 2000xp to advance to 2nd level, 4000xp for 3rd, 8000xp for 4th, and so on. An interesting effect this system had was that, if a player lost a PC, their new character was likely to "catch up" with the group fairly quickly even starting all over at 1st level. While those surviving fourth level characters need 8k over the next few adventures to level up, the new 1st level character can use his 8k share of those adventures to advance all the way up to 4th (provided he survives, which is much easier to do in an exploratory game than a combat game).
3E introduced a more algorithmic advancement - characters needed 1000xp total for 2nd level, 3000 for 3rd, 6000 for 4th, 10000 for 5th and so on. What this meant was that, as the party average level increased, a new PCs chance of ever catching up with the rest of them decreased. Not only did a 1st level replacement for a dead 5th level character stand very little chance of survival, that disadvantage would remain with the character for the rest of its career. What this did was pretty much force DMs to either allow players with dead characters to bring in new characters at the same (or close) level to the rest of the party, or risk either killing off the new characters over and over, or worse, lower the challenge level for the rest of the group.
By the time characters reached higher levels in earlier editions, say 9th or 10th level, there was no question of newer characters catching up with the group - when the rest of the party needs 100k or even 200k to level up, even the lowest level characters are going to catch up before that happens.
Which brings us to another interesting artifact of older editions, "Name Level".