Orius
Legend
Okay, a big one is XP. Specifically XP for treasure. Until 2e, treasure was the biggest source of XP, the general rule of thumb was that 1 gp found and recovered awarded 1 XP, usually once you got back to your home base. 1e also had XP values for magic items that were found and used, or you could sell the item and get the money XP as normal. Anyway, fights were something to be avoided because they weren't as rewarding, but had a lot of risk. This was especially the case with random encounters, because they had little treasure, could hurt the party a good deal, and their purpose was to keep the players from tarrying too much or adventuring carelessly.
Now 2e shifted to making the rule optional. The DM was given several options for awarding XP. But XP for monsters was the easiest and eventually became sort of a default. So instead of avoiding encounters because of their risk factor, players became more willing to fight. And the increased focus on combat became more pronounced.
Anyway the rules shift isn't too big, but they way players approach the game is.
Now 2e shifted to making the rule optional. The DM was given several options for awarding XP. But XP for monsters was the easiest and eventually became sort of a default. So instead of avoiding encounters because of their risk factor, players became more willing to fight. And the increased focus on combat became more pronounced.
Anyway the rules shift isn't too big, but they way players approach the game is.