New spells to use
Currently, paladins are like carpenters with only one tool: a firehose. While that metaphor may not exactly fit, Holy Light does fit in just about every healing situation we run into. There's a time and a place for Holy Shock and Flash of Light, of course, but our priority system boils down to "keep up buffs, use Holy Light." Priests have to focus on using spells like Prayer of Mending and their 51-point talents on CD, while druids have a variety of different ways to handle healing tanks and the raid. I think shamans cast Chain Heal or something, but honestly I only remember that shamans even exist when I'm scrambling around for my "BLOODLUST NOW!!!" macro.
Holy paladins are going to have to get used to a healing landscape where we actually might have a spell we should cast instead of Holy Light. While Flash of Light may save us mana and reduce overhealing, there is almost no situation in the game where a Holy Light wouldn't have done the job too. That's going to change in Cataclysm: due to the speed and potency differences between Flash of Light, Holy Light, and Greater Holy Light, it's entirely possible that we could use the wrong heal and our target could die. A new way of triaging targets is being introduced, and instead of simply choosing which target needs the Holy Light next, we'll have to choose what heal to hit them with as well.
We'll also have to figure out how to utilize our new spells in the most efficient manner, such as positioning ourselves in a key position to maximize the benefit of the AoE pulses from Healing Hands. We are currently very flexible in positioning due to Beacon of Light's 60-yard range, but now we'll have to be concerned about who we're near as well. That's something that we've never had to think about as a class before, and so it will take some time to get into the mentality of stacking up on injured targets to heal them simultaneously. Depending on how powerful Healing Hands ends up being and with the current strength of Seal of Wisdom, I wouldn't be surprised if holy paladins are delegated the task of healing while in melee range.
More survivability means less healing
While we obviously haven't seen the full spectrum of changes planned for Cataclysm, we have already seen several new defensive abilities added that should reduce overall damage to the raid. I can speak personally to a few of the changes that rogues are receiving, including the ability to reduce their incoming damage by up to 50% for 30 seconds, and also a new finisher that will allow them to heal themselves in times of dire need. As more abilities of the defensive persuasion are introduced, the ability of a raid to survive incoming damage increases. Imagine if all of your rogues and warriors too half damage from Marrowgar's Bone Storm for the duration of the ability? That would significantly reduce the total amount of healing necessary, and allow your heals to keep more raiders alive.
There have been very few fights so far in Wrath that have really pushed the envelope in terms of the amount of healing per second (HPS) required. Holy paladins play a role in that fact as well, since we are capable of such huge HPS values and yet Blizzard can't balance around the idea that every raid has a holy paladin present. Right now, any healer mix can realistically heal their way through any non-heroic content. That means that when a holy paladin's massive firehose of healing comes into play, it's many times more than enough to handle the incoming damage. Once our overall healing capability is brought to be more in step with the other healers, Blizzard will be able to turn up the damage to a truly challenging level as well.
Life pools are going through the roof
Another factor in the problem of balancing incoming damage versus healer HPS is the fact that HPS drastically outscaled the growth of a tank's total HP. In order to introduce enough damage to challenge a holy paladin's HPS, Blizzard would have bosses so strong that they would kill tanks in mere seconds. If we're capable of healing a tank from 0 life to a full green bar of HP, the boss has to be able to do the opposite in the same amount of time. By reigning in the Holy Light firehose and adding a massive health boost in Cataclysm, Blizzard can balance around that problem. Ghostcrawler has stated that they want players at less-than-full life more often than not.
I think that something as simple as doubling everyone's life but having a very small increase in raw healing on the way to 85 would solve that problem. Tanks would be walking around with 100k life, and it would actually take us several heals to fill up their bar from empty. By introducing the idea of a wounded tank, it allows us to actually monitor incoming damage and choose the right spell for the job, with going OOM or tank death as the penalty for making a mistake.
I remember a mod, way back in vanilla WoW, called something like Damage Graphs. It would show a graph next to each party unit frame, with a line indicating how much incoming damage they were taking. This made it really easy to figure out who to heal, as it also included a "seconds until death" timer, which let you know how long you could wait to heal them before they would die, based on their current incoming damage. Right now, that mod would be useless, as players are either taking such high damage that we never stop healing them or so little damage that it doesn't really matter when they get healed. The "seconds until death" timer would read 2 seconds or 60 seconds, neither of which allows for a very interesting decision. That's going to change in Cataclysm.
Planning heals around incoming damage and actively letting players go unhealed while handling higher priority targets will be the new triage mindset. We can't afford to just handle every little boo boo that the fury warrior gets with our best heal: we have to figure out if they're next in line to be healed, how much healing they actually need, and how quickly it needs to get there (trick question: we actually won't be healing warriors at all). It's hard to think of a healing strategy that doesn't involve slamming my Holy Light key as soon as a target is below 99% life, and I have a feeling that it may take some serious retraining for me to properly triage my group through the first few dungeons at level 85. I am confident, however, that it will make us better healers in the end, and that a more dynamic healing playstyle is something we've been asking for since Molten Core. It's time to eat our own worgen food, and as long as Blizzard allows me to plan a viable holy spec with Pursuit of Justice in it, I'll gladly take the first bite (werewolf pun not intended).
The Light and How to Swing It (Holy Edition) is dedicated to helping holy paladins become the powerful healers that we're destined to be. If you're new to the paladin's healing ways, you can learn the ropes with our Holy 101 article. We also have information on how to keep a tank alive, how to heal a raid when necessary and how to beat the GCD. Tanking is a job, DPS is a craft -- but healing is truly an art.