Here is a quick guide to playing an Arms warrior:
Mastery and Versatility are far ahead of the other two stat choices. Mastery is so much farther ahead of Versatility that sacrificing a lot of Versatility for some Mastery is often beneficial. This makes the World Quest Mastery trinkets BiS for Arms (except for Unstable Arcanocrystal).
If you want to play Arms you'd better play with a Focused Rage build. Here's mine:
Bounding Stride (may switch to Second Wind)
Compared to fury the Arms rotation will seem deceptively simple, I personally find it harder.
1. Colossus Smash
2. Mortal Strike
3. Focused Rage (to 3 stacks)
4. Slam (Concurrently with Focused Rage if Battle Cry is up OR if you have a large rage pool)
5. Warbreaker (only use if Colossus Smash effect is not up on target and Colossus Smash is unavailable)
Focused Rage: This ability does NOT share a cooldown with any other ability, you should be using it concurrently with your other abilities. You will need to get used to running two separate GCD-timers. The only time you should not use Focused Rage is if it will delay the use of Mortal Strike. Never use when you already have three stacks.
Execute: When in execute range replace slam with execute.
Battle Cry: This move makes the build. While it is up nothing costs rage, use Focused Rage as much as possible and always use one of your three DPS-ability options concurrently. Burn as much free rage as possible while Battle Cry is up because each 10 rage spent reduces the CD of Battle Cry by 1s.
Avatar: Only use Avatar with Battle Cry. It often lines up with every other Battle Cry, but Arms is so RNG-based that it often won't. Always wait to use it.
Die by the Sword: This is a REALLY GOOD defensive CD. It greatly reduces the damage you take for 8s AND gives you a 100% parry chance. Optimal uses are: weathering a raid-wide damage phase, covering up for screwing up a mechanic, or temporarily tanking bosses (the 100% parry makes you into a tank for 8s).
Commanding Shout: Go read my blurb in the Fury section, I'm tired of typing.
If you ever get one pray that it is Archavon's Heavy Hand. Mortal Strike refunding 15 rage turns the move into a 5-rage cost ability, if you use Colossus Smash before Mortal Strike then it becomes a rage generator. So amazingly good it's dumb.