The short version: Single-target threat is pretty solid, AoE threat isn't what it used to be, and certain talents are good on paper but not really worth taking right now.
Slightly longer, before-the-cut version: Basically, the redesign of rage and abilities like Heroic Strike/Cleave, combined with the total talent redesign, have had a significant effect on protection as a spec. While it's certainly a viable tanking spec, there is definitely a noticeable shift in tanking mechanics. Threat is once again much more in the hands of the DPS to take, rather than in the hands of the tank to lose, especially when talking about AoE threat. However, while the Cataclysm mechanics are already being felt, we are still in the Wrath era and many heroics will be tanked with DPS who simply will not wait for a tank to establish any sort of threat before unloading. You will lose threat in those situations. In some cases, even if the DPS does hold back, it doesn't matter. Their damage is so much higher than your threat generation that there is simply no way to prevent them from pulling aggro, and since the mobs die so quickly, they see little to no consequences for doing so.
This is a habit that will only be solved with the coming of Cataclysm dungeons that punish such behavior with repeated wipes. Until that time comes, grit your teeth, do your best, and accept that the fundamental balance has been altered. It may feel a bit like being a whole new class, but ultimately protection is still primarily about the same things.
4.0.1 brought changes to tanking
It's probably not the worst idea ever to go over how protection has changed.
- Fewer talents means some old favorites are gone or vastly different. Vigilance is not at all as we remembered it, Damage Shield is gone entirely, and you can't spec into arms for Deep Wounds effectively. Safeguard is still around, proving either that a merciful deity does not exist or finds joy in my confusion. OK, my traditional hate for Safeguard aside, a 30 percent damage reduction to the target of your Intervene isn't terrible; it's just not worth two points to get for an actual tank. If you're a protection PvP player, it's pretty nice. Shield Slam is now protection-only again and part of the talent specialization for picking protection at level 10.
- In addition to this, rage normalization and the change of Heroic Strike and Cleave to instant attacks have definitely changed a staple of warrior tanking up to now, which is to say, spamming on-next-melee attacks. In addition, changes to the way the game processes abilities and decides when to use them makes spamming anything somewhat problematic, depending on the situation. On the one hand, your wrist and fingers will probably be ecstatic about not having to hammer on the HS button like an enraged drinking bird toy -- but it definitely takes some adjusting.
- Some new talents will work a lot better once we're out of the Wrath mindset. I'm thinking specifically of abilities like Blood and Thunder and Thunderstruck. It's not that these are bad abilities (especially Thunderstruck) but rather that the current dungeon mindset tends to mean that in a 5-man heroic with four random strangers, it can be very difficult to get the time to use them, and as such, you feel constrained by the other options available. While I'm still using Thunderstruck in my current instance tanking build, I've dropped B&T for now and will probably pick it up again as I level. Pulls don't last long enough for me to get much use out of an ability that allows me to spread Rend around after using Rend on one target, frankly. By the time I hit Rend, then TC, I will have lost aggro. If I manage to hold aggro for a Rend/TC combo, it will tick maybe once before everything is dead.
Supporting the new paradigm
With rage normalization in; threat modifiers on abilities more or less gone in favor of pure threat being baked into tanking stances/forms (what this means is your tanking moves themselves don't do bonus threat, but rather your Defensive Stance provides all the bonus threat you have, making Heroic Strike and Cleave high-threat moves only in Defensive Stance itself); and some really powerful DPS abilities out there, the problems I'm seeing with protection as a tanking spec aren't the spec's fault. It actually has quite a few solid abilities for generating and holding threat. It's still a fun and dynamic spec with some of the best mobility of any tanking spec.
The problem as I see it lies in what I like to call the threat ramp-up period, which has existed in one form or another with all tanking specs since the game began. The current form of prot warrior tanking is much, much less dependent on ramp-up than it was when the game started or even compared to the Burning Crusade days. As things stand right now, with a Shield Slam almost certainly ready to go at the beginning of a pull or even being able to charge into a pack and Thunder Clap almost instantly, the problem lies not in getting the pull focused on you the way it used to be when the primary means of pulling was a ranged weapon.
One of the reasons Blood and Thunder ended up not being as excellent a talent for 5-man tanking is this ramp-up issue. Simply put, you have maybe 5 seconds in most 5-man dungeons to put out as much threat to as many mobs as you possibly can. With threat generation more normalized and passive threat reductions removed from DPS abilities in favor of increasing the threat on stances/forms to compensate, what ends up happening is that one spec that is currently a high DPS outlier often puts out a rather massive burst of threat. If you're not already above where it is going to be, the DPSer pulls threat off of you with ease. Here's a completely made-up example.
The difficulty of ramp-up time
Suppose you charge a pack, hit Thunder Clap and then Shockwave, and follow that up with a nice Shield Slam. You're low on rage at the moment, but you've done a solid 9k threat per second (assuming you managed 3k DPS during that pull, 9k TPS should be reasonable) on the mob you're primarily focused on and about 6,000 TPS to the other three targets in the pull. So far, pretty simple. Now, your fury warrior (I'm picking on fury here because of what I wrote about them last week and because, frankly, other classes like hunters cry and whine if you point out they're aggro bandits) has used Intercept to come up to the mobs at about the same time you did. As you were starting your attacks, he opens up with a Bloodthirst > Cleave > Whirlwind combo on the same mobs and generates 12,000 DPS on them. He no longer has any threat bonuses or modifiers; he's just doing 1 threat per second for every damage per second he's doing. So that 12,000 DPS burst is the same as 12,000 threat per second.
What this means is you don't have time for fun, fancy moves like applying Rend to one mob, then TC to spread it around like a DK disease. Heck, even DKs are saying there's not a lot of time to do that anymore. In my own personal experience, as long as I manage to get into a pull and put out some threat immediately before my DPS can unload, it's not really a problem, since those huge DPS spikes are often followed by lulls when I can generate steady threat and pull ahead of them. It's a bit like the tortoise and the hare, but ultimately it works out fine. If a pull lasts more than 15 seconds in a Wrath heroic, threat will be cemented on me anyway. And I will be surprised.
Gems and glyphs
In general, gemming hasn't really changed yet, except you can't use hit as your yellow gem anymore and you have to decide between hit and stam for a blue gem, which makes me sad. Still, for right now at least, your hit targets haven't changed so neither will your gemming strategies.
Your prime glyphs are absolutely always going to be Shield Slam, Devastate and Revenge. There is no choice to be made there. (I'd like to see a prime glyph that modified a defensive cooldown to give tanks a choice.) Major glyphs give you some more variety, with standouts like Long Charge, Rapid Charge, Resonating Power, Thunder Clap, Heroic Throw, Shockwave and Spell Reflection. It's in the major glyphs that you find the most room for customization and choice. I recommend learning them all and carrying some vanishing powder around to allow you to tweak on the fly, in fact. For minors, I usually just go with the Shout trifecta of Demo, Commanding and Battle, but if you've taken Impending Victory, then the glyph Enduring Victory is also useful.
To sum up: Protection's a viable tanking spec, but tanking itself has changed. If you go into it expecting to not have to work for your threat in those initial few seconds of a pull, you're going to be unhappy. DPS can easily rip threat off of a tank who isn't working to build it, especially if he plays with no regard for threat generation, and it won't be until Cataclysm that the dungeon design punishes DPS for this behavior. Keep your ego in check, keep working to build threat, and you'll find the spec is still a mobile tanking spec with lots of options for various situations.
Check out more strategies and tips for warriors of all specs in Matthew Rossi's weekly class column, The Care and Feeding of Warriors.