Main Caps

Theese are the main and most important stats to get. Without them your attacks will be missing, dodged, parryed, or consumed by armor, so getting theese caps will hugely improve your DPS.

See this Hit Rating?
Don't you ever do it!

Hit Rating

Hit Rating is the main cap for every other class in the game and Rogues are not exception. Without Hit cap your skills, spells and posions will simply not reach the target. Do you want it to happen? I don't, so I get the Hit Cap. But how many Hit Rating should we get? Short answer: 237 Hit Raiting or 9% in Magic tab of your Character Stats.

Why is it so and why most Rogues go for 315 Hit Rating and beyond?

Against lvl 83 boss in 3.3.5a spell Hit Cap is 17%. Oddly enough, poisons are considered to be spells which means that every Rogue has to get the 17% Hit Rating in Magic tab to not miss with the poisons. Combat Rogues get 5% Hit chance with Precision 5/5 so they only need 12% more from their gear or other buffs. For some reason most people just ignore that other buffs part and get the whole 12% or 315 Hit Rating with their gear and gems. Some will even recomend to go beyound that number to increase auto attacks accuracy. This is redundant!

Getting only 9% of Spell Hit with your gear is fine, because of Shadow Priest's Misery and Moonkin's Improved Faerie Fire. Raiding with these classes will decrease the chance of your poisons to miss exactly by 3%. At least one of them will be in your raid almost always if it's a 25ppl dungeon. It means that getting more than 9% Spell Hit Rating is useless. You'll have to significantly cut your other stats to hit 315 Hit Rating mark and it would serve literally no purpose.

But wait, what if there won't be a Moonkin or Shadow Priest with me?!
Well, first of all, there will be one in the raid, don't even worry about that. Second, even without this 3% hit buff, your poisons would miss like 3 times out of 100 hits so... Is it really worth to loose a bunch of ARP/Crit/AP that will increase the damage of your other 97 poison hits to get these 3 extra poison applies? I don't think so.

Expertise Rating

Like every other melee class Rogues need to get 26 Expertise to avoid lvl 83 boss dodging or parrying your attack from behind. In bare numbers it's 132 Expertise Rating with Weapon Expertise 2/2 tallent. The only thing to be careful about Expertise is, once again, overcaping. Most of the races in BiS equipment will need to get 30 Expertise Rating with gems and I urge you to not thoughtlessly put 2 Percise Cardinal Rubies into your equipment, but instead carefully calculate what other stats or gems colors you still need in order to get the exact number of Expertise that you need and not overcap.

Armor Penetration Rating

ARP has an complicated formula but it's a quite simple cap. You need al least 1399 Armor Penetration Rating to reduce enemy armor by 100%. But do not overcap! Get yourself as close to 1399 as you can. I recommend you to start building this cap right after you get Hit and Expertise caps. Don't listen people who tell you to ignore ARP and get AP or Agility while you're in the low gear. Your main sources of damage are auto attacks and Sinister Strikes which profit of the ARP. Upgraiding them will benefit your DPS.

Other Stats

Theese stats don't have any cap values. They're good to get after you already have all the main caps. With theese stats you have some options.

Crit vs Haste

Both are good and usefull. You'll get them mostly from your gear, however, after getting your ARP cap you can insert some gems with Crit or Haste. Crit is usually more preferable, because Glyph of Sinister Strike procs of crits. It has a cap that you probably never get, which is about 55% unbuff, but if somehow you get more close to this number - you can gem for haste.

Attack Power vs Agility

One AP gem will give you 40 AP whereas Agility gem gives only 20 AP, however it also gives some Crit and is amplified by the Blessing of Kings so I think it's still better to get Agility rather than AP.