Two numbers occupy my mind because I'm a tank: 540 and 102.4%.
540 is the Defense Skill cap, emphasis on Skill because there's also something called Defense Rating (which is the number you tend to see on Equipment, I need 690 Defense Rating to get 540 Defense Skill.) I need that much Defense because if I'm doing it right, the mob will constantly be hitting me. Mobs have a standard chance to score a critical hit on me. Having 540 Defense stops that from happening.
Having the Defense cap ensures that mobs can't score critical hits on me at all, means that my damage intake is steady, means that a healer can easily heal me.
Gearing up in Northrend from quests and the nonheroic dungeons, get a blacksmith to make you some items, maybe get one or two items because you have the right level of reputation with certain factions (Kalu'ak, Oracles or Frenzyheart) you tend to end up with somewhere around the 540 cap. You may be just below (happens with DKs a lot because they can't use shields) or just above (Excellent!) I found it was pretty easy to get the cap, but hard to keep it. Heroic/raid gear tends to have more Stamina and other usefull stats but not quite as much Defense. By gemming and enchanting correctly, I kept the cap while upgrading my gear.
The other number... That's the clincher!
102.4% is the unhittable cap. How does a mob hit me? In short, when a mob take a swing at me there are 6 things that can happen: Crit, Hit, Block, Dodge, Parry or Miss. Miss is a set rate that can't be changed (sometimes, mobs have a bad day and just miss me.) I've removed Crit already with my Defense so when a mobs swings at me it's down to 5 possibilities. Due to the way hitting works in WoW if I were to have have a lot of Parry, Dodge and Block that'll remove Hit completely as well. When Hit is removed, there's only 4 possibilities: mob misses me, I parry the mob's attack, I dodge the mob's attack or I block the mob's attack. So at worst, I'll block the attack! A blocked attack does 1k less damage than a straight hit - this is what tanking is all about.
I'll talk about this some more in a future blog as the consequences of these two numbers reach out to almost everything I do.