The job of a healer is a very simple concept. Heal the people taking damage. All healers have the capability of doing that and all can do it well, so what exactly could set one healing class ahead of the others?
For that we need to look at the three biggest things you can do while healing. At least the three ones people like to point out.
All healing classes are capable of doing this. As a healer you heal. As a healer you have heals. Nothing to see here really.
Keeping people from taking damage to begin with. Disc Priests are the king when it comes to this one aspect of the game. They can mitigate damage on the fly better then anyone else, but an earth shield is no laughing matter either.
Movement is something that you only control your own but it is a huge aspect of healing. Some healers have more movement ability then others but when it comes to movement as a part of healing it is more the movement of others that matter. If they move from the bad stuff your healing job is easier, if not, it is harder. You, sadly, have no control of other peoples movement.
What sets a Shaman above all other classes when it comes to healing a 5 man dungeon is that their healing tools extend way past healing.
Lets take a quick look of all the little things that a shaman healer can bring to the table in a 5 man that other healers can not. While they also bring these things to a raid it is often not as noticeable because it is not as needed but when it comes to a 5 man with pug people you know you will never have ideal players that know what they are doing. That is why the tools that are in the Shamans toolbox brings them to a whole new level when it comes to healing.
The single most powerful healing spell in a 5 man dungeon. It is also something only a Shaman healer can do. If it where this alone it would be enough to make the Shaman the king of the 5 man. But is not a healing spell you say. Sure it is, it is a huge healing spell, you are just not thinking of things correctly.
Take a spell like Whispers of Hethiss, it does some serious damage. It could take a good two or three large heals to heal the damage it caused. If you wind shear it instantly it does no, or limited, damage.
This saves you mana and the stress of trying to save someone. By stopping the spell you are stopping someone from getting hit for 50 or 60K worth of damage. Interrupting that spell is effectively a 60K heal for less then one quarter of the cost of a greater healing wave. Not to mention, it lowers your threat to boot.
Just looking at the math of it, a 60K heal (interrupt) for 8% base mana as opposed to a greater healing wave that might, if you get lucky, crit for 50K, for 33% base mana. Without a doubt, wind shear is your most cost effective heal as a shaman and quite possibly the most cost effective heal in the game over all.
Argue as you might that it is not the healers job to interrupt but you'll have to agree that it is the healers job to keep everyone alive. If no one else is interrupting, then it is in fact the healers job to interrupt. My last 3 times healing that boss I am the only one that has ever interrupted that spell because in 5 mans you need to remember most people will be really stupid and I can not heal stupid, but I can do their job for them and make my job easier in the process.
Bind Elemental / Hex:
The second thing that a shaman healer has that no other healer has is the ability to CC on the fly. They can not CC everything but a large section of things. Just like interrupting a spell is in effect a heal, CCing is also in effect a heal.
Many pulls there ends up being that stray caster that seems to love the healer. If the healer has to start spamming heals on themselves, you know things are going to go bad, fast. A simple 1.5 second cast for 8% base mana can quite possibly take care of your problem. Better to CC it then try to heal through it or hope that a pug tank will even notice that their healer is being ripped apart.
Sure, it can miss being it requires hit rating but it sure as hell is a better option to try when no one else is doing anything. That is pure mitigation. A mob that does no damage means no damage to heal.
Lets just forget that a Shaman can bring a whole slew of buffs to your group and being it is a 5 man it is most likely that some of those buffs will be needed. There are other totems that have huge uses.
Earthbind Totem: Slow a stray mob giving the person it is following more time to get away and the tank more time to pick it up. This helps limit the amount you need to heal.
Grounding Totem: Let it take that next big spell instead of you limiting the amount you need to heal.
Tremor Totem: Everyone just got feared, not any more, and in some cases this could be the difference between a wipe and survival.
Stoneclaw Totem: Stray mob after you, let it go after your totem while you get away, again, lowering the amount you need to heal.
Fire Elemental Totem: Burn phase? Help DPS. Dead mobs do no damage.
Earth Elemental Totem: Tank goes down, instant tank to the rescue. Better then your Mage tanking.
Searing Totem / Magma Totem: Might as well help with adds. Dead mobs do not damage.
As you see, many totems are in effect, healing totems but helping you have to heal less.
Shocks can play a huge part in making your job as a healer easier.
Frost Shock: Shock an add in GB giving DPS more time to get it down by slowing it.
Earth Shock: It lowers attack speed, does anyone actively remember that? It can be very helpful.
Flame Shock: Followed up by a Nova and you can do AoE too. The faster mobs go down the less you have to heal after all right?
You might not think of this as a healing spell either but I have had it save my butt more often then I can count. Lame tank not getting adds? Wolf out and bring the adds to him. You move faster, avoiding damage, thus requiring less healing. Ghost Wolf can be a healing spell too.
Last but not least... Heroism:
Okay, not a healing spell in any way shape or forum right? Well, not exactly. The faster a group gets a mob down the less damage it is alive to do. If something can go down even 15 seconds faster that is 15 seconds less healing you need to do. You can use heroism to help your team at times when it is needed or use it to help yourself when you know your mana is starting to run low.
In the world of 5 man dungeons with pug people the Shaman reigns supreme. They have more tools then any other healer, more control then any other healer, and is made to handle nearly every situation in the game.
Sure, if you are running with DPS and a tank that can interrupt you do not need to. If you are running with people that know how to CC you will not need to. If you are running with people that had decent DPS you would not need to worry about things taking to long to go down. But you are not running with any of those people, you are in a pug. If you are a Shaman, at least you can do it all yourself.