This might be too mechanical as well, but:
Skill Challenges. How they were presented in the DMG and, even more unfortunately, used in modules and LFR events made them a bit of a dud, but the base idea as originally described and used by (IIRC) Chris Perkins in a preview was golden. Using it as a "hidden" DM tool for adjudicating is where it shines, by presenting it to the players not as a skill challenge but just as a "what do you do now?" and then using the structure as a pacing, tension, and resolution mechanism. AND it works not only with skill rolls, but anything that could add towards solving the challenge, including powers, abilities, spells, assets, contacts, equipment, etc.
I've used this all over to simulate stopping rituals, traversing dangerous journeys, negotiations, creating battlements, and stopping weird situations, such as this (which came from Perkins' example): the players are trying and stop townsfolk from mind-controllingly walking into the lake and drowning themselves. One player might start grabbing townsfolk, another uses magic to try and shock/scare them out of their stupor, another climbs to the roof and parkours to get out in front of the townsfolk, while another wrangles a cart to create a barricade, and so on.
Less mechanical:
Clarity. This is an adjunct to Visual Design: I played an awesome dwarven runecaster from level 1 through 20. It was very evocative and effective, and it was all a re-fluff of the Artificer class. The clarity of 4e made it easy to take things and either repurpose them or rework them slightly to create even more options and flair and flavour.
We used this in an interesting way for some of our campaigns where the DM let us pick an RP race and a mechanics race; the mechanical abilities had to be explained in the fiction for how they presented themselves for our character. 'Dragonborn' fire breath on an elf? Could be an innate spell ability they learned in their youth, or maybe an additional ability of being a druid or paladin, or maybe they were a pyromaniac and always carried a little vial of alchemist's fire...
Though I never tested it, in this vein I would try allowing power selection from other classes without requiring the swapping feats. I might still gate it by requiring the initial multiclassing feat, but using the same 'justify/explain in the fiction' thing as noted above I could see it allowing for some really neat concepts, possibilities, and fun moments.