Why is a gap closer important?
The one huge quality of life difference between paladins and the other tanking classes was, for years, mobility. If there was a boss that required a tank to zip quickly from point A to point B, be it toward some critical add that just joined the fight or out of some obstacle that threatens to kill him, the paladin was always at the disadvantage. Warriors could quickly jump into action (or out of it) with their vast array of mobility toys. Bears can similarly charge an enemy. Death knights could bring adds to them, though obviously this was worthless on bosses.
Meanwhile, paladins had to slow trot over to the far-off target at run speed or a little above it. Probably while panting in frustration as the warrior gallops laps around him.
Consider a boss encounter like Al'Akir that requires tanks to be in range at all times, lest they be shocked with a channeled lightning attack. Then a wave of tornadoes comes rolling by, with the gap (of course) all the way in the back. You'd dash out, get zapped like a idiot jamming a fork in an electrical socket, and hope to get back in before the healers lost you.
And then there's the warrior who quickly leaps out to the gap in the tornadoes, then charges right back to Al'Akir, taking much less damage than the paladin.
Or consider a fight like Shannox, where you are taking a constant beating from the boss as his damage stacks higher and higher. As a paladin, your only hope is to slowly kite the boss while juggling cooldowns and sweating bullets any time Shannox is supposed to drop his stacks. Meanwhile, the warrior is doing pirouettes around the area, leaping away from his target at the last moment to buy some crucial extra space and guarantee that stack will come off.
The pattern here (other than that warriors are jerk-face show-offs) is that mobility can equal survivability in certain circumstances. I've said it before -- as tanks, it's our job to hedge against any corner case, and having a tool that would prevent (or at least aid against) getting squished in a mobility-dependent corner case is a huge plus.
The three choices
The first choice, and the one that I feel is the best overall, is Speed of Light. It's a controllable sprint button that increases your movement speed by 70% for 8 seconds, on a 1-minute cooldown. The obvious downside is that the minute cooldown means that it will be the most of scarce of the three gap closer options you have -- the trade-off, obviously, being that it's just so much faster than the other two.
Nonetheless, the biggest upside to this talent is its controllability. You can determine when you get the speed boost and thus not have to worry about wasting it (or at the very least, having it at an inopportune time). Combine that with the 70% run speed boost, and you have a natural winner.
That's not to say the other two talents are completely without merit. Indeed, there are times when they will be the better choice.
Long Arm of the Law will have a nice synergy with the Sanctified Wrath talent, theoretically giving you 20 consistent seconds of a 45% run speed boost. Otherwise, it seems better suited for PvP or leveling, for the purpose of giving chase to ranged targets. The rotation generally won't be flexible enough to recommend banking Judgment until you can use the speed boost -- not to mention, you'll be hurting your holy power generation by not judging as often.
Lastly, Pursuit of Justice (our old friend) takes on a very staccato form in Mists. Gone is the passive 15% run speed boost from the old version (and I'm mourning that pretty deeply). Instead, now we have a constant passive 10% boost, followed by an additional 10% per charge of holy power, up to 3. There are two ways for this to play out. One is that you're incentivized to bank holy power and keep at close to the 40% boost level as much as possible, or the other is to completely ignore any conscious decision-making related to the talent and play as normal, subsequently having to put up with sometimes going fast and sometimes running at a slight boost.
For the former, I can't see the value in that playstyle, as it'll essentially require trading mitigation (via Shield of the Righteous) for going faster. Doesn't seem like a fair trade to me. With regards to the latter option, you'll have to put up with what will feel like lag spikes. Sometimes you'll be trotting along at 40%; other times, it'll feel like you're plodding at only 10%. I don't know about you, but constantly suffering through highs and lows of run speed would drive me insane.
Ultimately, as I said, I feel like Speed of Light will generally be the best choice, especially for tanking boss fights. You don't need to move that often, so having a really powerful sprint at your beck and call seems like an awesome addition to the toolbox. It won't make us nearly as mobile as our fluttery warrior friends, but it'll go a long way toward closing the gap between the mobility of our two playstyles.
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.