A good amount of the WoW population spends the majority of its endgame time in 5-man dungeons. Blizzard is acutely aware of this, and it's striven to make new and engaging 5-mans. The 5-mans were more or less left alone in The Burning Crusade, with only one new dungeon during the entire tier, and the only major innovation was the wave system used in Opening the Dark Portal, which was later mimicked in Violet Hold and The Culling of Stratholme. In Wrath, Blizzard released four new dungeons over the course of the expansion and added in vehicle-based encounters in Trial of the Champion and the Oculus (which thankfully haven't been reproduced anywhere else yet). During Cataclysm, Blizzard has already released two new instances and announced plans for three more in the upcoming 4.3 patch. It's added interactive environments, randomized bosses, and some of the most complex 5-man encounters to date.
So what is Blizzard going to add next? Since Wrath, new raid tiers are accompanied by new raid meta-achievement mounts. While raid encounters are unquestionably more difficult, it'd be a nice nod to the players who can't raid for some reason or another to have a new mount to aspire to in 5-man content. A new Glory of the Hero mount for each tier of 5-mans that come out would be a good touch and a way to make 5-man content feel a little bit more connected to the storyline. Blizzard already implemented nine achievements tied to Zul'Aman and Zul'Gurub, so tying them into a meta wouldn't be much of an issue.
What would you like to see in the future of 5 man content?