Broken Steel did a rather decent job with showing the effects of the player saving the Purifier though, as well as the continuing struggle to drive the Enclave out of the Capital Wasteland... It wasn't much though.

However, the efforts it would take to truly show allt he effects of the players actions in a post-ending setting would be rather... big. Almsot too big to be worth the effort, probably. Maybe the same as creating two games in one <_< .

Yes, if a game just keeps going you get bored after a while

True enough, especially when all there is ingamewise to do is a couple of endless fetch quests and revisiting places you have been before. Although having said that, everyone has a different tolerance for just puttering around in an openworld game just doing what they want for however long they want to.

With regards to BlackSpider's comment about my Borked Steel method of referring to Broken Steel. I used that term mainly because the Broken Steel DLC for Fallout 3 completely shattered the games balance and made it far to easy to become what amounted to a wasteland deity and also far too hard to end up with multiple characters who stood out from each other for being different once you finished playing. Yes, it can be viewed as being very whiney, but conversely a lot of what has been said about not being able to continue playing has also been very whiney.

As I've said before, I find a lot of games where you can continue playing after you have completed the main quest, leave me with the feeling that the main quest line is basically shallow and meaningless in the grand scope of the game, to the point that no matter what you do as you play through the game to the 'end' of the main quest line it will not matter as everything after the end will be the same each time regardless of how you played through the game.
Now if Obsidian are going to be as thorough as I suspect they have been with the ending of New Vegas, then in order to create post ending content they would need to take into account a veritable multitude of possibilities happening during each playthrough, which to do well could very well take another year or two for them to do to their satisfaction.
Another thing to take into cognisiance with post ending content is that, no matter how many possible, plausible and frankly unlikely outcomes they can come up with is that there will be someone who comes up with a combination of actions during their playthrough that the devs could never consider happening. This happens all the time to PnP (Pen & Paper) Games Masters who run tabletop RPG's, in that no matter how carefully they construct an adventure, campaign or scenario the players in the group will come up with something that they never considered and or hare off in a completely different direction than envisioned one.