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Ok, lemme ask you this, from what I have read, it seems that I am to concern myself none at all with defense rating? I mean insofar as gemming or enchanting?
It depends. The thing with defense rating is that it lowers the chance that you get a critical hit on you. In heroics and raids, getting crit can one-shot you - which is why a proper tank will be crit immune before going in there. Which basically means that you will need to achieve that level (which is 535 defense SKILL for heroics and 540 for raids [=about 690 defense RATING]) before specifically focusing on any other stat. As long as you are not immune to crits, as a rule of thumb, you can avoid all other tank stats in favour of defense rating - which means that you might want to go for a +def enchant to reach that point. Once you reach the point where you have so much passive defense rating on your gear that you are crit-immune, without enchants or buffs, you'll want to forego all +def enchants you might have to get stamina, dodge and parry rating, or what-have-you.
Keep in mind that ALL of the above information only goes if you want to TANK. Like Dilbrt pointed out, none of this works for a DPS Death Knight. In fact, DPS Death Knights want to forego defense, parry and dodge rating completely and focus on completely other stats instead (like, for example, armor penetration). If an item has defense, parry or dodge rating on it, again as a rule of thumb, you will NOT want to use it if you are DPSing. If you want to swap between tanking and DPSing, you will need two different sets of gear to do it effectively (and probably two different specs too).
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Also, from what I've read on the most currently updated thread on EJ, my number one stat to stack for is Str. If I understand correctly (again, lotsa numbers and stuff) Str is better than AP because Str will grant AP. Also, is there a limit or a cap on what my ArP should be? Again I appreciate the info.
The cap on armor penetration is technically 100%, though the problem is that armor cannot be reduced below a set amount (around 3200, I believe) AT ALL. Which means that once you reach, say, 70% armor penetration, there is little use in getting more of the stat seeing as the difference between 70% and 90% armor penetration is going to be minimal.
That said, you'll want to read over these two links;
http://www.wowwiki.com/Defense
http://www.wowwiki.com/Armor_penetration
They have more detailed information on how defense and armor penetration work.