Completed the first mission in KZ Mercenary and found that the items you purchase in the campaign mode carry over to the multiplayer. Now it makes sense why so many people had more equipment in multi on day one.

You can veer off the path in campaign and find extra areas to "explore" with enemies/items/people to rescue. The first stage felt pretty solid, albeit a bit easy on normal. I think I'm going to try replaying it on veteran. Multiplayer has been great so far.
Leveling up is not like CoD, where it nets you better weapons and attachments, but rather gives you access to more loadouts. Everything appears to be available right from the start, but you have to earn money through campaign/multi to unlock it all. Besides the usual array of guns and grenades, there's different types of body armor available for purchase, plus the van-guard system, which deploys different types of mechs to assist you and other types of CoD killstreak-like bonuses.
My biggest complaint about the game is the touch screen elements. It's great that you can turn off a lot of it, but it's still forced when doing melee and when turning valves/arming remote bombs. It felt pretty awkward planting bombs on a tank during one battle, stopping to use both hands to rotate the image on the screen, while enemies were running and shooting at me. Ditto for attempting to use stealth and sneaking past an entire area, up to the door, only to find that it need to be hacked -via a touch screen mini game- and the time it took caused me to get noticed and shot up.
This is pretty much the first Killzone game that I've really sat down and played (I tried KZ2 at launch and hated the controls) and I'm really, really impressed with the former SCE Cambridge Studio's efforts. They've done such an amazing job in creating a console-like FPS experience on the go.