The AI was cut due to there being too many bugs with it, I think that's been officially stated by Bethesda, I just can't remember when/where.
As for other cuts, I can't think of any. Placing down tons of trees and making river systems doesn't take up ginormous amounts of space, in fact it just takes up a few KBs, maybe even a MB or two. I've done it tons of times. I think they just didn't feel like making Cyrodiil the way it was described.
If you're talking about the E3 demo with the voices, they were probably cut for "rumors" because only a few combat noises and unique phrases don't actually cost that much space at all. But they probably wanted everyone to have a rumors topic with random things, and it would cost too much space for tons of voice actors to do that. It isn't the voices that did it, IMHO, it's the rumors and every NPC "needing" to be able to say exactly the same thing when having conversations with each-other.
If they recorded only combat noises, some unique dialogue for each person, a few phrases like "Really? I never knew that." and a rumor or two, it wouldn't cost so much space. Then they could have a few voices that would be for "rumors" and the "speakers" in random conversations like that and would only be made for more "generic" NPCs like beggars, guards, and some civilians. It isn't the voices that did it, it's the fact that they had to have each voice actor say so many things instead of just a few things. They had random NPCs wandering around in Fallout 3 that said nothing, or rude things and left, which upset many people. It would have been different if every one could have said a unique greeting and you had the option to ask for "rumors". However, in Fallout 3, despite those NPCs, it has been improved upon, it just isn't perfected yet.
What I'm hoping for is if they have a few "generic voices" like they did in Oblivion, but then they combine it with the "unique" ones from Fallout 3. This also improves modding because then you can voice-record something without sounding like a Main Quest character.
Summary
If every character doesn't have Rumors, we'll be fine with space for the next game.
In fact, they could have added TONS of characters using the method of only recording combat sounds, and a few strings of dialogue for NPC and Player interactions... it would have cost LESS space than having everybody in Oblivion say the same things. They would just need combat sounds (a bunch of short yells and mean phrases), one or two "Who's there?"s, "Hi I'm ____, I do ____ for a living"s, and then a few lines of dialogue for things like quests, or just for diversity...