The Crimson Binome
Hero
A lot of it might be that 3E started to standardize things a lot more, and they standardized around the idea that "above average" characters were about average for PCs. In 3E, you could reasonably expect that your PCs would have a Con bonus to HP at each level, where older editions would vary wildly from table to table. (Also, since they flattened out the ability modifiers, you would get more from having a 14 or 16 than you would under the old methods.)Healing in combat is a 3e thing because the monsters hit so much more often and so much harder than their AD&D counterparts due to the idea of CR=4 PC's of a given level. A troll in AD&D maxes out at about 25 points of damage, if it hits every time. A 3e troll has twice the HP, a better AC and maxes out at over 50 points of damage if it hits 3 times. IOW, a 2e troll has to fight for 2 or 3 rounds before it can whittle a PC down to single HP. A 3e troll can flat out kill a 5th level PC in a single round.
In my experience, an AD&D troll would kill a 5th level PC in 1-2 rounds, because few PCs had more than 4-5 hp per level and they were probably going to hit with at least two attacks. If you had better stats, or more magic items, then it could easily tip the scale in the other direction.