So, I think Bounded Accuracy and Advantage/Disadvantage are the key winners in a poll. But the way that 5e has added support more support and means of roleplaying your characters is probably my personal favourite. It's little thematic touches all through the races, classes and features - random details, trinkets and more.
In particular, the way that the old Prestidigitation spell was used as a template to make Thaumaturgy and Druidcraft, so that any character playing one of the big three types of magic user can now pick something and be able to do iconic magical things with those classes. Not powerful, but extremely thematic.