From what you've seen so far - what would you say are the strengths of 5th ed as far as campaign design? How should someone like me build a game that showcases those strengths - that says "this campaign works better with these rules than with the old ones?"
That's tricky, because IMO the biggest strengths of 5e are:
1) minimal core size: the mandatory rules for resolution of interaction/exploration/combat are very few (mandatory in empirical sense, i.e. if something can be ignored/unused by the gaming group without changing characters balance or requiring other alteration to the game, then it's not mandatory even if the rules don't specifically say it's not); everything else is modular
2) complexity in the hands of the players: options exist in different parts of the game for increasing character complexity quite a lot over the baseline, but it's done at individual level
3) not many assumptions on the gamestyle, although not every possible gamestyle is easily supported, but at least the whole game isn't built around a single "right" way to play it, unlike the past two editions
How these strengths can be showcased? Well, #2 depends on the players, so it either happens that they make wildly different characters or not, but you can't choose in advance unless you force them to (which is kind of against the point itself); and #1 and #3 become visible across a number of campaigns with different DM's decisions for the whole group (e.g. magic availability, combat/skill/adventuring/inspiration modules) but how do you highlight them in just one campaign?