You know, I was watching the E3 2007 trailer again the other day, just for the heck of it, and I have to be honest, there were a lot of things in that trailer that I wish they had kept for the final game. I'm not entirely sure why this stuff was cut, but I'd imagine making the game co-op had something to do with it. There were some things they talked about in early previews and interviews, too, that didn't make it in.
If you watch the trailer again, you'll be able to see a lot of this stuff (though the Collector's Edition strategy guide elaborates on a lot of it). I can't help but get the feeling that, once they decided to make the game co-op, the developers felt like that was all they really needed to do. It was the only "innovation" necessary, as it were. I strongly believe that they limited themselves conceptually as a result. Obviously, I can never know if the version of the game depicted in that initial trailer would be a better game than the version we got, but I still think some of their ideas were interesting and would certainly like to see them return in a future title.
The enemies were apparently more intelligent than they are in the final game, as they could pick up random objects from the environment and use them as weapons. This is evidenced by a screenshot from the early build in the CE strat guide where a Majini is attacking Chris with a chair. The enemies could also tackle Chris and even pin him to the ground (you see this in the trailer), perhaps holding him down so the axe Majini could decapitate him?
Chris himself apparently had more abilities at his disposal, as well. He could dodge regular enemy attacks with the press of a button (you see him ducking a couple times in the trailer, and the CE strat guide confirms this was done by pressing the Action button at the precise moment). He also seemed to have counterattacks, which presumably were to replace Leon's boot to the head when breaking free of a Ganado's grasp. You see Chris punching a Majini and kicking some away after pushing them off in the trailer. The melee attacks themselves are still in the game, but you can only use them when a Majini is stunned. Otherwise, Chris just pushes the Majini off.
One of the biggest things I wish they had included in the final game though was destructible environments. There are still some in the game, but few, if any, actually affect the gameplay in any real way. They're more set dressings or scripted events than anything else. In the trailer, you see Chris standing on what appears to be a makeshift wooden balcony of sorts. The axe Majini then swings his giant axe, taking out the supports of the balcony, causing it, along with Chris, to come crashing down to the ground. There's really nothing like that in the final game, sadly.
Perhaps the biggest aspect of the gameplay to not make it into the final game (at least not in the same sense as it was originally intended) was the light and darkness aspect of the environment. Takeuchi, as well as other members of the development team and Capcom, stated on several occasions that they wanted to recreate that sense of moving from a dark area to a bright one and vice versa where there is a brief period of time where your eyes have to adjust before you can see anything clearly.
You can see this in the trailer, though it's perhaps not as obvious as some of the other gameplay mechanics I've described thus far. The idea was that when Chris moved from a darkened room or building out into the bright sunlight, you wouldn't be able to see anything for a few seconds while Chris' "eyes" adjusted to the light. The development team was going to use this brief period of time as a means of "scaring," or at the very least, surprising the player, by having enemies jump out and attack while they couldn't be seen.
This is still in the final game, but in a limited capacity, and it's never used for gameplay purposes. The most obvious example from the final game is when you just start, before you meet with Reynard and get your weapons. If you enter one of the shacks and look outside, it'll be extremely bright, and you won't be able to make out much of anything. That's about it, though.
The game was also apparently going to be much more open-ended and exploration-oriented than RE4 was, at least according to early interviews with Jun Takeuchi. Ironically, the final game turned out to be even more linear than RE4. I imagine this was a direct result of making the game co-op.
There was one element discussed early on that was never actually intended to be included in the game (it was a mistranslation from an interview with Takeuchi) that nevertheless could have provided an interesting twist on the "survival" portion of "survival horror," and that was the idea of overheating. It was believed that Chris, if he spent too long out in the open and in the bright sun, he could potentially overheat, and this would affect his performance (aiming, for example, would become more difficult). So players would have to seek shade or cooler areas to let Chris cool off. Drinking water was even mentioned.
Again, while this was never meant to be a part of the game, I think they could have done some interesting things with it. Some speculated that overheating would cause Chris to hallucinate (similar to an early build of RE4 where Leon had hallucinations as a result of being infected with some sort of virus). Obviously, this isn't something that would work too well with the co-op gameplay. Still, I think it would be interesting to see in a future title. I imagine it would be akin to Naked Snake's stamina gauge in MGS3 and Old Snake's stress meter in MGS4.
Anyway, as I said, maybe we'll see some of these elements return in a future title. I certainly wouldn't mind seeing them, myself.