Wow really? No thread going for this game already? If you haven't played it yet it's a great game. The story is based around terrorists using BOWs this time, not Umbrella (are they finally gone for good now?). Most of the game takes place on a cruise ship crawling with monsters.
I beat the game on easy already, and haven't gone very far in the next difficulty (god I love New Game+) as I'm distracted by Raid Mode. Raid Mode is smaller sections of the main game with extra enemies thrown in. Your performance is rated on Accuracy, Time, Damage, Number of Enemies kills, etc. The better you do, the more BP you get to spend on new weapons and upgrades. This mode can be played single player with an AI buddy, or online with a real person. Playing through this mode gains you XP toward levels, but I'm not sure what levels for you as you dont really see increase in health, damage, or anything. It may unlock higher level items in the store.
Missions are extra incentive to play certain levels. As you play you will receive missions that are usually like "Kill 20 Hunters" or "Reach Level 30". These give you new weapons, BP, and items. The more fun missions you get come from playing online with other players. Say you do a couple levels with "Waughoo". You might get a mission to go to a certain level where you will find Waughoo has become one of the monsters and you might kill him. Once you get to level 50 (I'm still in the 30s) you can get Infection Missions that have bigger rewards. Infection missions are passed from player to player, so once you're infected you can infect others.