You can drive just about every vehicle you encounter in Crysis. Chances are good that you won't often want to get behind the wheel, however, as smart enemies typically turn them into sitting ducks. Stay in a truck too long and chances are awfully good that it'll be blown to bits along with your nanosuit-wearing self.
North Korean Trucks
A variety of trucks can be found scattered around the North Korean camps and patrol locations on the island. The most popular model (aside from unarmed civilian pickups) is a jeep-like half-track with a gun turret mounted on the top, while you also frequently come across larger semi-style trucks with similar roof guns.
Neither is much use for driving, unless you want to cut down on travel time. Each get blown up pretty easily by enemy grenades and gunfire. It's best to just use these vehicles as gun turrets on wheels. Hop in them, go right to the main gun, and use it to blast any nearby KPA goons. Then get out before the North Koreans turn it into your coffin.
American and North Korean Tanks
Both an allied and an enemy tank can be driven in just a single level in the game, the tank-rush special called Onslaught. There isn't much to choose from between the two models. The American tank seems to be faster, while its North Korean counterpart seems to take more damage, although this is only a guess given how little time you spend in them. Both are pretty nimble for video-game tanks, meaning that they're fairly easy to drive unless you get all Evel Knievel on hills.
Just like the tanks described above, an American VTOL can be flown in just a single mission, in this case the penultimate Ascension. And thank heavens for that. These clunky aircraft are like the Yugos of the sky. They turn painfully slowly, have pathetic top-end speed, aren't great with inclement weather, and don't stock much in the way of missiles. Just like the KPA's trucks, you're best off not trying to move a VTOL very far. Use it as a floating gun turret. Or better yet, just fly straight ahead as fast as you can and cross your fingers.
If any aspect of Crysis feels skimpy, it's the rogues gallery of bad guys you get to shoot. The only foes in the game are North Korean soldiers, North Korean assault choppers, and two different types of alien. Great artificial intelligence and intense battles mean that you don't notice the lack of variety too often, thankfully.
Although they come in different uniforms and tote different weapons, the average Korean People's Army (KPA to their friends) soldier remains the same throughout Crysis. Each seems to possess the same intelligence and desire to kill the "American pig" invading his turf. Look for them to be outfitted for different situations accordingly. In levels set deep in the jungle, expect KPA patrols to be decked out in full camo outfits, while bases are staffed with grunts and officers in regular uniforms.
Generally, the KPA carry pistols, FY71s, shotguns, and SMGs. Some are scattered throughout the game with body armor that makes them harder to kill (although all North Koreans can take a pretty crazy amount of punishment before going down). And in select late-game levels, such as in the tank mission Onslaught, you run across KPA goons tricked out with missile launchers.
KPA Nanosuit Soldier
You only encounter these North Korean knockoffs of yourself on a few occasions in Crysis. A couple of them first show up in the cemetery at the end of level three, Relic, but most can be found in level six, Awakening, at the rock crusher and in the mine. The later nanosuit-wearing KPA also come equipped with miniguns, making them formidable opponents. They never seem to take much advantage of suit functions like cloak, however, which leaves them open to counter-attack.
KPA Assault Helicopter
A regular pain in the posterior, KPA choppers show up to harass you at numerous times throughout Crysis. They track your movements closely, forcing you to keep cloaked and crawl through the jungle. Remain visible to helicopters for too long and they'll rip you apart with machine-gun fire or incinerate you in seconds with a missile. Only handheld missile launchers do a good job of blasting these whirlybirds out of the sky, although you can take them out with miniguns in a pinch.
Only encountered on the alien ship in level seven, Core, these presumably organic creatures are fast and deadly. Some seem to shoot ice pellets at you, while others take the fight up close and personal and attack melee-style. You can be killed instantly by these rushing attacks.
Thankfully, these aliens often ignore you. Give them a wide berth on their ship and they simply go about their business for the most part. Only engage them when you absolutely have to, as you're at a serious disadvantage in the weightless alien ship.
These man-size mechanical fliers with robotic tentacles and bug-like faces are the shock troops of the alien army. They can shoot ice bullets at you from and do heavy damage with their tentacles. Most bullets plink off of their metal carapaces, although they can be stopped in their tracks by shotgun rounds. Speed is their biggest weapon, as they can leap on top of you and tear you to shreds before you have a chance to react.
Hunters, which look a lot like robotic octopi, are the toughest opponents in Crysis, with the exception of the few boss monsters encountered at the end of the game. They look and move a lot like the drones that sliced up Morpheus's ship in The Matrix. Their biggest characteristic is speed, as they slither through the air very quickly and can fire both a freeze ray that can kill you in your tracks and a machine-gun-like barrage of ice. When these mechanical menaces get close to you, they can rip you apart with their tentacles, Doc Ock-style. Swarms of them can be found the later levels of the game, providing you with some extremely difficult battles.