The following chart is accurate most of the time. It is not accurate to every situation or cirumstance.
*Enrage requires will not be effective unless you have Resilience gear. This is covered in "Effective Stat Allocation."
Mitigation Threat/Damage Rage Mgmt. Utility
Anticipation Defiance Focused Rage Imp. Thunderclap
Imp. Shield Block Shield Slam Imp. Sunder Imp. Demo Shout
Toughness 1h Spec. Anger Mgmt.
Deflection Shield Mastery Imp. Heroic
Shield Mastery Devastate Imp. Bloodrage
Imp. Def. Stance Cruelty Unbridled Wrath
Enrage* Imp. Bers. Rage
Unranked (Unable to Compare)
Tactical Mastery; Last Stand; Imp. Revenge; Imp. Disarm; Imp. Taunt; Imp. Shield Wall; Imp. Shield Bash; Vitality. Take these abilities whenever they are useful to you, and remove talents from the bottom of trees you don't need to get them -- for instance, if your Threat is fine, but Imp. Shield Wall would be a big help, cut Cruelty out.
Imp. Sunder with Devastate
Imp. Sunder is going to save you considerably less rage if you are actively using Devastate. If this is the case, move it down below Imp. Heroic.
Shield Mastery rated above Devastate?
Yes. For tanks who have collected a decent amount of Shield Block Value, Shield Mastery does more for added Threat than Devastate does for added Threat over Improved Sunder -- this is a very fine distinction, because the effectiveness of Devastate has to be factored against the rage saved from Imp. Sunder. Roughly one in four to one in five uses of Imp. Sunder over Devastate -- for average tanks -- will yield an additional Heroic Strike (this assumes that the Heroic Strike itself will yield no rage, thus penalizing rage development).
Complicated, sure. Short answer? Shield Mastery is better than Devastate, but not by much.
How does Shield Block Value affect the above charts?
Shield Block Value is unique in that it scales separately from other Warrior abilities or stats. Blocking for 400+ Damage on a creature only dealing 500 damage is going to be far more effective than blocking 400+ Damage on a creature dealing 5,000. For this reason, you will need to reconsider the balance of Block Value on your own, based on the creatures you are fighting.
However, with 400+ Shield Block Value, your Threat output dynamic is going to change in a predictable way. Shield Slam and Shield Mastery will outperform Defiance in +Threat potential. Most strong Protection builds will have both talents, so take it for what it's worth.
Picking Your Talents
The reason I divided the above charts the way I did was to make it clear that there are several different things you are doing when designing a talent tree. With Protection, there are no cookie-cutter builds, which sets the Protection Warrior apart from most other classes.
Pick your talents based on what you need. If you need more Threat, start looking into talent builds which produce more Threat. If your Threat is fine, but your Mitigation is terrible, work on that and get the Utility talents which help.
There are times when Imp. Taunt and other various talents will become very valuable to you -- if they are, get them in your build. You are absolutely not required to take any specific talent if it doesn't help you in your tanking environment. Cruelty, for instance, is a great talent in a great place, but it never has been, nor ever will be, a requirement of tanking.