Seems to me like Bungie is trying to move from Waterfall-style development to a more Agile-style development approach. In other words, instead of doing a bunch of big up-front design for the game and each expansion (which they still do some of I'm sure), they are moving towards an approach where they are constantly updating the game.

(Assumptions below, we'll see how it pans out in reality)

One possible example of this might show up soon in the "Queens Wrath" event. I'm thinking that will introduce additional "story" type missions that take place in the same existing geometry. It's not a full expansion because of it's limited scope, but it still adds new things to the game for players to do so they keep coming back. Plus, because Bungie has set up the game to be easier to adjust and update on-the-fly they can keep an eye on how things are working out for players and make game-balancing decisions without releasing a whole new game. Halo Reach had a similar setup where 343 was able to make some adjustments to the gameplay after release. Destiny has even more capabilities for adjustments.

A lot of people have been asking questions about why Destiny took so long to develop. I think it's because they've been building a lot of foundational architecture to support this kind of stuff. In addition to building a new engine that is capable of running on multiple platforms, they've had to build a lot of support tools to let them update the game much more dynamically than in the past.

I didn't mean that. Destiny was advertised as a huge epic experience focused on exploration. We were told that the Solar System was our sandbox and shown concept art of several locations on Earth, along with Saturn, Europa, and derelict ships. I'm sure we'll visit all of those places eventually, but showing them to us as part of the marketing for this game heavily implied that they would be included in this game. And that's what a lot of the marketing was like, in retrospect; Bungie heavily implied many things without directly stating them. I guess they figured they could get away with implying things and then, when people complained about the missing content, they could point back and say "We never actually promised any of that!"

(Okay, I'm being slightly unfair. It wasn't all Bungie. A lot of it was Activision.)

The final game provides four playable spaces and offers mostly linear (and mostly identical) Dinklebot escort missions. It feels small. In fact, it has about as much content as a standard FPS game, not the epic quantity that was heavily implied in the marketing. It is not the experience that was advertised.

I never expected an epic Mass Effect style narrative, especially not with the open mission structure that I envisioned for the game. I suspected that much of the "meat" of the story would be told in supplemental media (i.e. the Grimoire). However, I did think we were going to get something more complete than what we ended up with. Almost every story point of interest, along with all of the back story and character motivations, is buried in the Grimoire.