Auras are an important part of a Paladin's arsenal but are also the most underrated. If I had 1 Copper for each Paladin I've seen rush headlong into battle with Crusader Aura on, I would probably have enough Gold to train for Artisan Riding all over again. Auras are passive area-of-effect buffs that affect the Paladin and her party and cost nothing to cast. Because it is a party-wide buff, it helps to get the Holy talent Aura Mastery for more group utility. Auras can also be toggled in between eating and drinking or while mounted, but not while casting spells. As good as Auras are, however, it suffers from one major shortcoming: it activates the Global Cooldown.
The GCD is the biggest reason why Auras aren't toggled more often during an engagement. It slows an already slow cycle down. Played right, however, the right Auras at the right time give subtle benefits to a fight. Too many Paladins toggle one Aura and neglect to change it up throughout the course of the fight. One reason is because of the GCD, which will set back a spell or attack cycle by 1.5 seconds. For healers, this delay can result in a casualty because virtually all of a Paladin's healing spells are cast. For Retribution, this results in a drop in DPS, and for tanks it already adds to the numerous cooldowns they need to juggle.
On the other hand, removing the GCD from Auras entirely opens it up for abuse, so the most sensible option would be to have Auras on their own cooldown separate from the rest of a Paladin's spells. At the moment, all Paladin abilities share the same GCD with the exception of Judgement, Divine Favor, and Divine Illumination. This design issue aside, however, Auras give Paladins a passive little extra and learning how to use the right Aura at the right time is crucial to playing a Paladin properly.
This Aura is the standard for most healing Paladins, particularly in PvP or when grouped with casters in a raid. Concentration Aura one of the most useful Auras and can be made more effective with the creatively-named Protection talent Improved Concentration Aura, a must-have for PvP healers. Because Paladin healing is highly susceptible to spell pushback, Concentration Aura gives that extra protection -- in addition to juking, of course -- during crucial moments when the Paladin needs to get a life-saving heal off. As good as it is, this Aura isn't as desirable in 5-mans where the Paladin is less likely to be engaged by mobs.
Hands-down the coolest, most utilized Aura in the Paladin's arsenal, Crusader Aura is the fastest mounted speed enhancement in the game. It doesn't stack with items such as the Skybreaker's Whip or the soon-to-be-craftable Carrot on a Stick nor to the Retribution talent Pursuit of Justice, but it remains the best mount enhancement despite this limitation. Because the +20% speed increase is applied per 100%, it scales immensely with faster -- or epic -- flying mounts, up to a 456% increase, arguably faster than taxis. Paladins must learn to toggle to another Aura upon dismounting, as it is often embarrassing to be told to switch Auras before the first pull.
This is the starting Paladin's Aura, and serves her until max level. Ironically, because the armor bonus is fixed, rather than a percentage, Devotion Aura is more useful to cloth-wearers than it is to tanks or plate-wearers. Even with Improved Devotion Aura, which confers an additional 344.4 armor at Level 70 (for a total of 1205.4), it isn't terribly useful for tanks who have anywhere from 14k to 20k armor. It also suffers from the mechanic that a tank, in theory, will be taking most of the hits, rendering the shared effect useless to other members of the group. To this end, most raiding healing Paladins take Improved Devotion Aura while Tankadins take points in Redoubt. On the other hand, Devotion Aura is extremely useful in PvP when fighting against Warriors, Rogues, and other melee classes, often giving Paladins just enough mitigation to outlast opponents when toggled at opportune times. Devotion Aura also gets a boost from the earliest Libram a Paladin can get, the Libram of Truth.
Of all the Auras, the resistance Auras are the most situational and should be toggled between the use of other, more useful Auras such as Concentration or Devotion. The biggest pitfall of resistance auras is that they do not stack with other, similar buffs such as Fire Resistance Totems or Mark of the Wild. Since their number is fixed and not a percentage, they do not scale very well, although they do stack with gear. Resistance auras are excellent for encounters that are heavy on a particular element. In PvP, it also helps to toggle Auras right before getting hit by particular spells such as Frost Resistance Aura against a Frostbolt, Fire Resistance Aura against a Pyroblast, or Shadow Resistance Aura just for when Warlocks are running amok.
Criticism has been levied against Retribution Aura for its shortcomings in scaling (see a pattern with Auras?) as it doesn't scale with +spell damage, but is rather good when taken in the context of Area-of-effect tanking. Against single targets, the Paladin is probably better off using Devotion Aura or one of the resistance Auras. Against multiple mobs, however, Retribution Aura is synergistic with Holy Shield and Blessing of Sanctuary, producing high threat and damage. It is also a good choice for 5-mans with no other Paladins, although the threat generated by the damage from Retribution Aura is attributed to the player getting hit, not to the Paladin. It also benefits from Improved Retribution Aura, which isn't a particularly desirable talent but pushes Tankadin DPS and threat against multiple, fast-hitting mobs. The 2-piece set bonus from the Tankadin Tier 5 set also confers a bonus to Retribution Aura.
Finally, we have the Retribution talent Sanctity Aura. Usually taken for Improved Sanctity Aura which grants a party-wide increase to DPS by 2%, it is unique in that it is the only Aura that scales, as it works off percentages. . Unimproved, Sanctity Aura only really benefits the Paladin herself, as very few classes rely on the Holy school for damage (lolsmite). It is a key component for the Shockadin spec, as well as assists Tankadins in generating more threat seeing as how they are the one class that deals Holy damage in abundance. Because of this synergy, Retribution Paladins can find a spot in raids in either the tank group or a (usually melee) DPS group. Highly niched, some raid groups find flat the 5% crit increase of a Feral Druid's Leader of the Pack to be preferable. Three talent points are a high price to pay for its benefits, but played right, Paladins who spec to take advantage of it can dish out some hurt.
Taken together, Seals, Blessings, and Auras comprise the core mechanics of the Paladin class and playing the class right requires a mastery of the use of all three components. Judicious use of Auras is the most underrated of all Paladin skills, and toggling Auras between spellcasts (yours and your opponents) is a difficult skill to master. Despite the GCD, it's possible to swap Auras constantly, with care taken to avoid locking yourself out of key spells at crucial times.
If there's anything I would like to see in the expansion, it would be to see the three core mechanics work with more synergy, such as having bonus effects when a particular Seal, Blessing, and Aura are activated all at the same time. It can probably be confusing to a degree, but given the rote style of play the Paladin class entails, the possibilities with these core mechanics are exciting. Here's to hoping that Blizzard is cooking up something for the original classes and not just disease-ridden gimmicks and air-conditioned dragon condos.