Halo 3 is ok but there isn't a very distinct difference between it and Halo 2
You'd be surprised.
Not amazingly better, but the game does look pretty good for how many objects it has. Amazing graphics is simply impossible with Halo's scale, and high-detailed levels can be a hindrance to multiplayer.
Assault Rifle is back (full of win
)
Yes, it's back, but it sucks, but works as a starting weapon. A shallow clip (32 rounds, or about 4 seconds of fire) makes it necessary to use grenades or a melee to tip things in your favor at close range, and spraying the fire will work well at medium range, but it seems to run out of bullets at infuriating times (that's why I always grab a secondary, even if I don't really want the weapon, and to cancel reloads).
the helmet changing thingamajigger
You're referring to armor permutations, which are unlocked through achievements in the game (I got all 49 achievements in about three weeks, but don't expect to do that unless you're hardcore). It changes more than just the helmet, it changes left shoulder, right shoulder and body.
There is no video file, just a data file with each person's action in a game that is played back through the engine to create the saved film playback (which has some sweet free-camera and third-person camera options).
What it really is is an object editor. The map's geometry stays the same, but in-game objects like boxes and explosives, as well as weapon, player, and objective (like the flag in CTF) spawns. It's also fun to mess around in, as you can go online with it with 8 people and have wars dropping tanks on eachother.
(if that wasn't already in Halo 2, I dunno)
They weren't.
The basic gameplay is the same though, which isn't a bad thing, just not the greatest.
I'd say its a nice mixture of Halo: CE and Halo 2, and radical gameplay changes just aren't a good thing for a shooter as popular as Halo.