Favorite and Least-Favorite changes in 5E
Favorite: Bounded Accuracy. The influence of this feature cuts right through everything else, enables somewhat-similar playstyles across character levels, and sets the tone of the whole game.
Honorable Mention: Background Features. Conferring language, skill, and tool proficiencies through the Background mechanic was a great innovation, though it was not as pervasive an influence as Bounded Accuracy.
Honorable Mention: Flavored versions of Neo-Vancian spellcasting. Wizards select a spell to cast from a prepared list, while Bards select a spell to cast from their small list of spells known; and other variations.
Honorable Mention: "Finesse" weapons. PCs do not need a Feat, nor do they need a Class Feature, to use their Dexterity modifier with certain weapons.
Honorable Mention: Advantage/Disadvantage. It is a "nice-to-have" feature in my opinion: without it, the game would still work as intended, but would not work as smoothly and simply.
Least Favorite: Mega-Feats: They are kludgey, in my humble opinion, compared to the careful precision of selecting feats in 3E and 4E. Wizards of the Coast could have easily kept Feats the same size as before by making "Ability Score Improvement" be a +1 to one Ability, and making them appear twice as often in the Class tables. There was no need to create Mega-Feats.