When logging at combat logs recorded from an encounter, one of the easiest metrics to judge a paladin tank by is their efficiency with maintaining that heartbeat. As I wrote in a column back in October, you can measure your heartbeat efficiency with this formula (you can see an explanation of it at that link):
heartbeat% = (total Crusader Strikes + (ShoR whiffs/2)) / (fight duration/3)
The higher the number, the more efficient you were at hitting Crusader Strike and thus generating holy power, and ultimately, how efficient you were at your threat output. The heartbeat is the fulcrum on which the whole rotation pivots, and if you're not doing your part, you'll find your threat will be weak as a result.
2. Using cooldowns at the wrong time When evaluating how to use cooldowns, the obvious instinct is to use them when you're health gets low in order to buy healers more time to get you topped off. While it's a noble instinct (and one that will likely save your life from time to time), it's also decidedly not optimal.
There are two immutable rules to tanking cooldowns that come into play here. The first is that damage prevented is always better than damage healed. Damage prevented is free; it disappears into the nether and troubles no one. Damage healed required mana to correct. It uses healer resources, and over a long enough period of time, it's unsustainable. Moreover, if you eat a massive spike and sit at a low health percentage, the healers will need to expend a lot more mana via their most expensive heals to quickly get you back to a safer level.
Secondly, the biggest threat to a tank's life is the damage spike, rather than trickle-down death by a thousand cuts. As such, you want to use your cooldowns to counter spikes as best you can. Learn the fights, the abilities the boss will throw at you, and the moments when damage taken will ratchet up. Then plan accordingly with your entire toolbox.
Following that plan of attack (well, defense) will repeat better results to a wider range of circumstances than fumbling to mass the Guardian of Ancient Kings button after eating a massive spike. And your healers will thank you for it.
For more on planning cooldown usage, I can't recommend enough this blog post by Meloree over at Sacred Duty.
3. Not treating Holy Shield/WoG as a cooldown With their short cooldowns and minimal investment, it might seem like a great idea to use Holy Shield and Word of Glory just about as often as you can. However, if you're seeking to gain the most impact possible from their use, the trick is to instead use them at the right time.
Just like Divine Protection or Guardian of Ancient Kings, Holy Shield and Word of Glory need to be used proactively like any other cooldown. Is there a large period of melee/blockable damage (for example, Baleroc's Inferno Blade) incoming? Pop Holy Shield to shave off a hearty percentage of the incoming damage, right before the onslaught hits. A huge magic burst (like Nefarian's Electrocute) being cast? Fire off a Word of Glory right before it hits to lessen the blow, thanks to Guarded by the Light.
4. Gearing for threat There are some major caveats to this, but generally speaking, a major mistake people make is gearing, gemming, and enchanting for threat over survivability. The impulse to cap your hit chance and soft cap your expertise is a strong one. Everyone hates seeing that accursed MISS pop up or watching Shield of the Righteous whiff four times in a row.
Nonetheless, our first job as tanks is to foremost bring as much survivability as we responsibly can to the table. This means that you should emphasize boosting (obviously) your mastery over any threat stats. Then once at cap, I'd still strongly recommend going into stamina (or avoidance, if that's your flavor) instead of seeking the threat caps. With the nerf to threat, there's generally little reason to seek out higher DPS numbers at the expense of your own personal survivability in a raid environment.
However, like I said, caveats abound. Heroic 5-mans, for example, don't require absolute optimal gearing. Unless you're still in blues, you can likely get away with capping your threat stats. Likewise, even on certain raid fights, there is a high reward with gearing for threat (heroic Ultraxion with its ultra-tight DPS check is one).
The key is to responsibly evaluate your approach to the content you are working on, not just strapping on DPS plate pieces, hoping for the best, and gawking wide-eyed at your big numbers.
5. Not using your Hands proactively I just wrote a column about this, but it still bears repeating. In group play, you are doing a major disservice to your teammates by not making frequent use of your various Hand spells. These should be keybound, macroed, and seared into your brain so they can fly out with nary a moment's hesitation, for maximum effect. The difference between a paladin tank who puts Hand of Sacrifice on his or her co-tank and one who doesn't is huge. It can mean the difference between life and death in corner cases, and isn't that what tanking is all about?
The Light and How to Swing It shows paladin tanks how to take on the dark times brought by Cataclysm. Try out our four tips for upping your combat table coverage, find out how to increase threat without sacrificing survivability, and learn how to manage the latest version of Holy Shield.