The Call of Duty 4 demo we were shown, instead of just showing a snippet of the game here or another bit there, gave us an excellent overview of the game's first mission: a helicopter infiltration, which drops you and your squad off on a ship crawling with soldiers during a storm.
The key to Call of Duty 4 is its atmosphere. Sure, the gameplay is solid - but it wouldn't matter if the developers didn't completely nail cinematic presentation and hyper-realism that has become the series' trademark. From what we've seen thus far, COD4 aces it. From the first dramatic moments you're pitched onto the rocking, rainy ship to infiltrate it till the harrowing escape from its rapidly sinking decks... it's non-stop action (well, probably the part they skipped over had some "stop" in it. We'll find out when the game comes out.)
It's difficult to describe what happened with accuracy - it went by fast, as the player seamlessly infiltrated the different areas of the ship with his squad and, eventually, tracked down the mission objective: a set of documents. Soon after, a detonation sent the ship into a wild angle - it's sinking. The dramatic finish was a dash back out of its twisty corridors and onto the deck, to jump onto the helicopter at the last possible moment. We also got a sneak peak at some shooting action in the Middle East.
The final interesting detail is a scoop on the multiplayer: you'll be able to create your own class, this time around, with access to abilities and weapons. It sounds like you'll specifically be able to tweak things (such as gaining "increased sprint") and then take your tweaked class online for multiplayer battling. This could add a whole new angle to the game - but balance will be the key or it'll just screw up everything. We'll see.
To check out the gameplay demo for yourself, look below: