I'll second the "challenge" point made upthread.
D&D used to in many ways resemble a rogue-like computer game: you'd make your character, put it in play, and let's see how long it lasts and-or how well it does. The game world/setting is out to kill said character and said death could come at any moment from any source; so when said character dies, lather-rinse-repeat until one sticks. Survival was job one and priority alpha; and therein lay the challenge. Luck played a very big role and bad luck was a challenge few if any could overcome.
Now the baseline expectation is that you'll finish the campaign playing the same character you started it with. Survival isn't a priority except in unusual circumstances. The challenge is both reduced and morphed; now largely being based on finding the best builds (bleah!), tactics, or story results to suit the in-game situation. Luck's role has also been greatly reduced, and bad luck can in many ways be mitigated or papered over.
The one element that hasn't changed at all (despite some who might say it has) is the act and art of roleplaying a character, giving it some personality, and making it both memorable and entertaining; in part because doing these things does not rely on mechanics.