Not only do the in-game characters look great - Drake in particular was constantly changing his facial expression to reflect what was going on around him, particularly when he got hurt - but your enemies are surprisingly smart. Not smart enough to keep from charging our position and letting us mow them down (or in one instance, to stop standing around smoking when a grenade went off nearby), but smart enough to pull back further behind cover when we drew a bead on their heads. They're also deadly enough that just charging around unloading on them is an excellent way to get killed by their crossfire, even if you do have a few cool martial-arts moves to bust out when the action gets too close-up.
Instead, you'll want to duck and cover until you've got a clear shot. Drake can flatten himself against pretty much any wall you find with the press of a button, and can leap and roll between cover points a la Gears of War. Of course, aiming means you'll need to leave cover, but you can squeeze off a few blind, unaimed potshots around the edge if you just want to lay down some covering fire. If you're lucky, it'll keep the bad guys from trying to get past your cover to outflank you.
In the demo, the hero was able to carry around three weapons at a time: his pistol, a handful of grenades and either an AK-47 or a shotgun. Trying to hit anything with the assault rifle was pretty dodgy, as it sprayed bullets so wildly that aiming for heads with it just netted us a couple of weak body hits, if we were lucky. It actually became much quicker to just switch to the weaker pistol, which nets instant kills if you can bring it to bear on enemy faces. We also had no problems with the shotgun, which didn't really have to be aimed at all to take down pirates at close range.
The grenades were similarly iffy. They didn't seem to do much damage unless they landed right next to our targets, and aiming them was extremely difficult when we were taking heavy fire from multiple bad guys - you know, the times you need a grenade the most. It was frustrating, but given developer Naughty Dog's dedication to fine-tuning in its previous games, it's extremely likely that any problems with weapons will be cleaned up before the final version.