So I will start off by saying that I started with Halo 3. I was a slacker and never had an XBox before the 360. Also I am a slacker and didn't get halo for the PC.
That being said.
Halo 3 - 8.5/10 Great story, Great Graphics, Great improvements to the FPS genre. I like the sticky grenades, the melee, and vehicles. Shields vs health bar. Yes I realize some of these features were in Halo 1 but like I said I'm jumping from UT99 to Halo 3.
As far as movement its a standard FPS forward, backward, strafe, jump, shoot. It would have been nice if they had the side roll(dodge) like UT99, or Gears, or some variations on the standard movement, but what do I know. This is why I like Gears of War they changed the overall game play of the FPS/TPS by adding a cover system as flawed as it may be. At least they gave us something new.
Halo Wars - 8.5 / 10 Best RTS ever made for a console. Great story. great graphics, and thank you for choosing radio menus. Might I mention how horrible most RTS menu systems are (Battle March == WTF). I was not really big into RTSs but this was a great game. Kinda short but well executed.
Halo ODST - 9/10 Yes I think it is better than Halo 3. Awesome storyline. I was a little confused at first on the flashbacks but once I got into it, it was great. I love the "alone" feeling they captured in playing the rookie in the streets at night, with just a few patrols of covenant.
Stamina + Health bar makes sense
In response to other posts
No shield.. duh! - ODST != Spartan
Can't jump as high.. duh! - ODST != Spartan
Can't punch as hard.. duh! - ODST != Spartan
No dual wield.. ok - ODST != Spartan
Ripping off turrets.. Yeah I don't get that either but ok.
Firefight is what Horde in Gears should have been. The reinforcements come from logical locations instead of just popping into existence.
The scoped pistol is new to me (didn't play halo 1) and the scoped silenced SMG is new to me so I will include those as good things.
usable banshees in campaign is awesome. although I wanted to fly the Phantom.
The Gauss cannon warthog.. very nice (is that new, I have never seen it before. I saw it in Halo Wars but not Halo 3)
What the heck is a grunt doing with a rapid fire Fuel Cell Cannon? The Grunts were pretty bad a--... well as bad a-- as grunts can be.
To me this felt like a new game, short, but new.
My few bad comments
TOO SHORT... I want more.
Still no new improvements to game play.
1 new useless enemy well two if you count those dome things that blow up when you shoot them. Never got close enough to see if they did anything else.