If Bungie can pull this off it will be quite impressive.

The larger the engagement and the more 'options' the AI/player has the harder it is to program. In Killzone 2 you have fewer AI players in smaller areas with fewer optimal paths. In Halo 3 you can come at them in any number of ways. In any one battle you could be on foot flanking or driving up the middle or in a Warthog or Wraith or Banshee from any number of directions.

In Halo: Reach almost all if not 100% of the engagements will be dynamic. This means the AI has to do what it has to do without guidance from the developer. If you consider the range of scenarios the AI has to face it has to be smart. There could be up to 4 players playing on the same system and the AI still has to reach appropriately to each of them.

LoL you do know that the video that the guy posted on Killzone 2 is on easy/normal right?

Try playing it on the hardest mode, the AI is clever than ever! Cleveriest AI i ever encounter

I agree, we all know PC can handle way more AI streams then consoles. Videos were cool though.

Still, I don't see 200 individual AI steams. I could see pre-done battle scenes in a very far off distance where scripted AI destroy each other.

"Elite Difficulty"

"Elite difficulty is the only difficulty that removes the crosshair during regular movement, and becomes available only in iron-sight." 

No, it's not the easy/normal mode. Unfortunately the AI is bad even on the Elite mode.

pf.....if someone here thinks that the KZ2 AI is bad, it's a complete idiot, try the game in Elite Difficulty and you'll see how bad the KZ2 AI is...

Halo 3 A.I is really, and i mean, REALLY good too, but there's no way that the KZ2 AI is the "Dumbest"...

In Elite Difficulty, they get you under cover the whole time, you can't run and gun, they kill your partners throw grenades, they flank you, they use their environment in advantage...EVERY enemy soldier is like a boss....

That video IS the Elite mode lol. And the flaw is clearly there.

You probably didn't pay much attention but try playing on Skirmish mode and see how the AI React, they play like real players.

And didn't KZ2 win the AIgamedev.com AI of the year award?

"The combat AI that Guerrilla Games built over the past 7-8 years since Shellshock: Nam '67, including Killzone 1 for the PS2 and Killzone: Liberation for the PSP has arguably become the best in industry with this latest iteration. The enemy behaviors are dynamic and adaptive, showing off both linear sections and large open areas of the game thanks to its tactical reasoning.

The AI in KILLZONE 2 in particular shines in the Skirmish mode where you can play with computer-controlled bots in large open maps designed for multiplayer, and free of scripts and trigger boxes. The bots were intended for offline play where you have 7 bots on your side against 8 enemies, but you can also compete online with your friends against various combinations of bots. Not only were the bots fun to play against, according many reviewers including to Ryan O'Donnell of Area5.TV, but they even passed an informal Turing test with journalists from the CO-OP show (season 2, episode 1) and others:

“The bot thing is pretty amazing. I was telling you earlier that I was playing it and it was behaving in such a particularly clever way in this one area that I had to check that I wasn't play online.”
— John Davison, WhatTheyPlay.com

Some reviewers even speculated that Skirmish and multiplayer bots were included simply to emphasize the strength of the combat AI. You can watch some High Quality videos of matches against the bots in the Salmun Market or the Radec Academy.

Cooperative buddy AI that accompany you through the game (while actually fighting alongside you) is generally a hard problem that nobody has got entirely right to-date, and there's still room for improvement in Killzone 2's buddies. However, it's one of the best attempts yet and given the underlying engine there's even more potential. In particular, there's a balance to be found between the aggressiveness that can save you in tricky situations vs. buddies playing safe so you don't need to revive them too often.

The Rest

See the rest of the awards in detail at AIGameDev.com!

you're wrong he's playing it on easy... he just said "elite" just to have more views, the comments in the video said that he's playing on easy.