Defense comes first -- defense rating, for tanks, allows you to make sure that bosses can't crit you. Each class has its own defense cap, and the cap has changed from 70 to 80, so you'll have to keep an eye out for your own cap when you get that far. Block is next -- a high Block rating means you're pushing other attack options off the table when you're hit, so that when something does hit you, you block the damage on it. Both of those stats are mainly for tanks -- other classes, who aren't getting hit, won't have to worry about them at all.
But Hit and Expertise you will have to worry about if you're DPS -- Hit will make sure that you don't miss your target (the fewer misses you have, the higher your DPS), and Expertise makes sure that those hits don't become dodges or parries. This is tough stuff, and it shows up much earlier in Wrath, it seems, than it did in Burning Crusade, But the good news is that there's a lot of help around -- Honor's Code offers a great overview for what everything means, and from there, you can search our site or others for what you need to know about each stat and how it works with your class.