Being a shaman, like the other hybrids, means that you end up picking a role and dedicating your time to it. You spend your talent points in one of the trees, run instances, PvP, and raid for gear to supplement that role, and you find yourself with lesser viability in other roles. My enhancement shaman can still cast healing spells, yes, but they're nowhere near as effective as my resto shaman's heals. Meanwhile, my resto shaman can still put Windfury Weapon on his Hand of Eternity, and often does when healing in a BG. This should not be taken to mean that he can actually melee as well as an enhancement shaman, and the bonus spell damage on his healing gear does not make up for the gear and talents, much less experience, of an elemental shaman.
This is more or less how it should be, but I think it's an error to ignore the other specs too much. One of the biggest mistakes I made when I first started playing a shaman was to focus too tighly on the enhancement tree and not looking at the others: as a result, I was fairly ignorant of how shaman healing worked, an oversight I paid for later when I had to start healing when I went resto. Meanwhile, in my guild there was another shaman who refused to spec out of the resto tree even though he wasn't healing any groups, meaning that he leveled exceptionally slowly. (He was level 30 when I started my first shaman, he's still not 70 yet long after I reached 70 on my second.) You can level resto, certainly, but you should look into a few points in elemental (in my opinion) if you're going to do that, just to give your shocks and spells some more bite and lower mana cost.
Having come from a background wherein I play mostly melee characters and am often tanking, switching to a 'true hybrid' like a shaman means that I have to start thinking in terms of collecting entirely different sets of gear for the express purpose of being prepared in case I'm asked to respec to a different role, or in case I'm needed to fill a different role than usual in a fight. Sometimes bosses are not melee friendly, or they do a lot of burst damage that can be hard to heal through, and it's better than I stand back and do ranged DPS or heal. It's helpful to carry a healing and caster set of weapon/shield at the least to help out in those situations, if nothing else. (Sometimes you don't have advance warning and can't swap gear ) I main healed the last boss in Heroic Arcatraz when our priest got a crit heal at exactly the wrong time and went down in my enhancement gear, which, by the way, is not terribly fun. You throw down your totems, slap on your healing weapon/shield, and grit your teeth in frustration trying to keep everyone alive with the healing and mana pool of an enhancement shaman, I'll tell you now. Amazingly, as resto, I once had to step up and kill a mob when our mage went down and I had to keep flame shocking the scarecrow, and in that case I still had to keep healing the raid.
Some shamans like to keep their points spread out in order to emphasize the hybrid nature of their class. Frankly, this has always struck me as an awesome idea that the itemization doesn't do nearly enough to support. (This is not me saying that all shamans should be able to go 41/41/41, not at all, just that it would be nice for groups if a shaman could bring more hybrid viability to offset the things they lack, like CC.) There's some nice synergy between the low level resto talents and an enhancement build, for instance... it's almost impossible to imagine an enhancement shaman who hasn't taken Nature's Guidance, for example. As a contrast, a starting elemental shaman might well begin raiding with 20 points in resto like so, and then throttle back Nature's Guidance as his or her gear caught up on +spell hit. But it's hard to see much in enhancement within those 20 points that's going to appeal to an elemental shaman, and as enhancement I always felt that Convection and Concussion, while attractive, ultimately just got in the way of getting up to Nature's Guidance as fast as possible.
I should admit that I took them both on my first shammy, huh? Yeah, I didn't even look at resto, more fool I.
In the end you'll probably spec deep in a tree. I know that, as much as I sing the praises of a broader spec, I tend personally to go deep in whatever spec I've chosen - this is my preferred enhancement build (except that it doesn't have Totemic Mastery, which is stupid and which I just dropped the G to fix), while this is how I roll restoration. (Yes, I know my gear sucks.) I freely admit I only went that deep into elemental to get Elemental Warding, a talent I simply don't want ever to do without as a healer. This is what I do when I play around with elemental, as we've already seen above. I freely admit that my elemental fu is not strong... there's something about being a caster DPS that I can't wrap my head around, it's even more alien to me than trying to be sneaky. But I've seen some truly excellent elemental shammies and I keep trying to steal their wisdom.
One of the reasons to collect gear for all three specs, and to familiarize yourself with all three, is to ultimately be of more utility to your guild once you start raiding. If your guild has all the melee DPS in the world but is short on healers, then as many shamans before you it may be time for you to choose to be restoration to help them progress. A good guild will recognize that your heart may not be in it (even if you turn out to be really good at it) and will allow you to collect elemental or enhancement gear as well... heck, often there's not really another shaman along and you end up getting it anyway. Likewise, maybe your guild has all the healing they could ever possibly need and are interested in putting together a caster group with a mage or two, a moonkin druid, a warlock, and you in your elemental finery collected from various instances. Elemental shamans can help provide nice totem buffs (Wrath of Air and Totem of Wrath at once is a very nice combination, and mana spring doesn't ever hurt) as well as their own excellent DPS. Keeping current with the three talent trees means that you can offer your guild the flexibility that playing a shaman is really all about.
My last piece of advice is, if a piece of gear is just going to be sharded anyway, don't be shy about saying 'Hey, I could use that for my X set.' If you have a piece that serves the role already and is equivalent then by all means, let folks have their shard. But if you're wearing a green Nagrand quest helm for your elemental and your restoration sets and a nice blue caster hat drops, especially if no one else wants it, don't let it go to waste. It's hard to gather gear for three complete sets, and you don't want to anger the rest of your party by taking the pieces they need all the time (especially when they're not going to be building off-spec sets, like rogues and mages, as an example) so never let stuff go to the disenchanter without considering your offspec set needs. You may think you'll never go enhancement, but if a 2.6 speed offhand fist weapon drops and no one wants it, get it anyway. You are not going to see a lot of those drop, and if you ever do want to go enhancement, you'll wish you grabbed it.
I now turn the floor over to you, gentle and not so gentle readers. What is your favorite spec? How up to date are you on the specs you don't usually play? (Yes, I know I need to be better about elemental.) What advice would you have for a shaman switching roles? How did you go about collecting off-set gear?