For some fights, you're required to stay away from eachother (empowered shock vortex etc) but there are plenty of encounters where you don't, and so many ranged (especially hunters!) are convinced they have to stand way out in Africa. I see this in 5-mans all the time--the melee are together, the warlock is a couple feet away from me, and the hunter is at complete max range.
@Chainawol. Thanks for mentioning discipline's fantastic capability as raid healer. I was starting to get frustrated how this thread was ranking them better tank healers than raid healers. All the early Professor Putricide logs showed disc to be number one on meters when you include absorbs.
However to say that disc is more versatile than shaman healers (at least in 25 man raids) is not really true. Sure disc has a lot of spells, but in all honestly you're also just doing 3 things like a shaman is: PoM on cooldown, shield spam, penance when people drop too low.
I think the point is when you're assigned to one job you are going to be limited to the spells you use, it only really becomes interesting when you attempt bosses using less healers.
Disc priests can be good raid healers, especially when there's predictable damage. On BQL we tried our usual set up (shaman&disc on a tank each, druid on raid) and realized there was waaaaay a lot of raid damage. Each tank was taking barely more damage than the raid. And the raid damage was constant and predictable--perfect for disc priest and resto druid. And then I just hung out and CH'd the two tanks. It worked out great! (...except that second air phase, where half our raid died and I was the only healer left up with two tanks and two DPS and we just burned... burned... and then I died and ankhed... and we barely survived with the tanks both at 10%. Epic first BQL kill)
I personally have no doubts that a Disc Priest can effectively raid heal, everyone can. The question is more of if they're better than another option. If your raid is sitting on, say, two Paladins and another Disc Priest, then someone's going to have to raid heal. Obviously, if someone has a Holy offspec, that's ideal, but if not, then you go with what you have. Disc Priests can do it, but they're far from optimal for it. There are a lot of raid leaders out there who discriminate against Disc because of its poor meter performance, to be honest, I wouldn't bless those raids with your presence. Someone also said that Resto Shamans only really shine as raid healers when there's massive incoming damage, which is somewhat untrue, but I would prefer to have a raid healer who shines in that situation than a Disc Priest who's spamming shields, simply because they don't have an efficient answer to that damage.
On the topic of the Resto Shaman, everyone has differing styles, but fundamentally, as Resto Shamans sit more or less in the middle of the raid-to-tank spectrum, their role is less one of focus and more of generalization, and as such, can be quirky to play. On some fights, this is absolutely invaluable, such as Festergut. On others, the Shaman takes a back seat to your Paladins and Druids in pure healing output, and instead should be focusing on problem areas. In this role, the Shaman relies a lot more on the Waves than on Chain Heal, focusing on ensuring people survive to be topped up, rather than on doing the topping up himself.
For example, someone taking the disease on Putricide for a lengthy amount of time is ideal for a Resto Shaman, who can convert from whatever he was doing on seeing health dip to a quick Riptide/HW combo or NS/HW for an even quicker response, and then follow it with spammed Riptide/HW/CH cycling or LHW in order to keep him up to transfer off the debuff. Because a Resto Shaman is going to be casting those HW's at 1 second or so with a maintained Tidal Waves, he's the best option for quick, large heals (aside from pulling a Holy Pally off a tank, which is a bad idea), making him ideal triage for when situations start to slide.
On top of that he has one of the quirkiest and most powerful heals in the game in Chain Heal. Chain Heal has the bizarre reputation of being seen as class-defining and yet underrated at the same time. The reason for that is primarily due to its long cast time. Many beginning Shamans have issues with that cast time, particularly because their Haste is so low, but also because their sense of timing and positioning are off so as to not get the maximum benefit from CH.
Remember that CH is a strong single-target option in its own right, largely due to unintended bounces. My classic raid strategy is to target our tank, and spam the crap out of him. It's not uncommon in this situation to get unintended spill over onto the melee, as well as providing solid tank healing. As has been stated before, the melee is the ideal place for CH, but it's far from its only use. Even without a bounce, it will be enough healing to save people, and it'll give you a fresh Tidal Waves, enabling a quick HW or LHW for more output right afterward. As explained in the post above, CH also functions as a lesser version of Beacon on dual-tank fights, such as Marrowgar and BQL.
Its drawback is the abysmal cast time, but for such a powerful heal, it's justified. No other healer would put up with a 2 second cast time for their most frequent heal. Because of it, Shamans have to develop a much more keen sense of timing than anyone else, both in anticipating damage (for instance, when someone gets spiked on Marrowgar, hitting Riptide to make sure the target stays alive, and then Chain Healing will likely catch the melee rushing to the spike) as well as its interaction with other heals, most notably the other two powerhouses of raid healing: Circle of Healing and Wild Growth.
Another Shaman tool that is drastically underrated is Earth Shield, and it almost sickens me when I see Shamans failing to maintain it. Coming from a Priest, the comparison is simple: You PoM on cooldown because it's that good. Earth Shield has zero chance of bouncing to someone who doesn't need healing. Why would you not make sure it's up at all times? It's not usually going to save a tank's life if he really desperately needs healing, but it's a huge mitigation tool on par with Rejuvenation and PoM.
Finally, do not confuse your meter's "effective healing" tab with actual effective healing. A solid 20-30% of a Resto Druid's "effective healing" isn't saving anyone's life, it's topping off, something that is occasionally a necessity, but most of the time is just meter farming. Chain Heal does this too, mostly on the third and fourth bounces, because the healing at that point is getting so low as to be almost ineffectual. Remember that the key to healing is not to generate the highest output, but to keep people alive. The Shaman has a fantastic group of heals for keeping groups alive, in Ancestral Awakening and Chain Heal, as well as strong single-target options, both on the fast side and the slow side. While he can't output single target numbers like a Paladin, the Shaman has the capacity to fire a quick, solid heal or a slightly slower (or as fast with Tidal Waves) massive heal, which is something that most healers can't compete with, and in doing so, also frequently generates AA procs to bleed his healing over to other roles. That bleed makes him an exceptional swing healer, filling whatever role needs to be filled.