Things are subject to change... but such drastic changes don't strike you as being a bit odd?

I don't see a lot of drastic changes to the game. They made quite a few smaller changes here and there. It's still a scifi fps shooter with MMO style aspects which is what the game always advertised itself as. They might have wanted to add some features but then found out they don't really work in practice or they were simply not feasible. To give an example in one of the videos they show the rocket towers getting knocked over for a public event on Earth. It looked really cool to watch. Great idea. But wait, public events happen multiple times a day and the specific one in question happens every 30 mins. How does it work that they always get knocked down every 30 mins but then are standing back up for the next one. The only logical thing would have been for them to be standing the first time and never be there again after that. It's not feasible for things like this to happen multiple times a day and it would look strange and break the persistent world model they have. Which change do you see as being the most drastic?

Story elements, mostly. Cut scenes from old vidocs and the dialog from them. I see that as being a drastic change. Changes made due to balancing issues, technical limitations, and (as you mentioned unprovoked) persistence I understand. Changes to the narrative I can't. Especially when this game was in development for... what? Five years? Granted a lot of that was most likely a lot of technical stuff, laying the groundwork and such, but I can't imagine writers in such a large team having anything to do with those things. I enjoy the gameplay. I just get the sense that there used to be a much different story and I can't fathom a reason why that would change so late in the development cycle. I'm not trying to be a tin-foil hat wearing idiot, but its a lingering feeling in the back of my mind when I play this game.

Story is only a small part of the overall game and relatively easy and quick to change. The game's story was not in development for 5 years. Most of that time is taken to build the engine, to build the UI, networking and gameplay. Author's and story writer's make changes quite often to restructure things. It's possible that Bungie decided to restructure the story to make the overall story flow better. I don't know for sure. In the vidocs there are only a couple of small cutscenes that we've seen. How do we know the story wasn't worse before than it is now? Movies usually have multiple endings or they go back and reshoot large sections to change the story about to get better reviews after test viewings, it's not really a new practice.