(disclaimers: I have no experience from current 20s to draw upon (yet!), and little experience with the other two specs, but I definitely can sell shamans in PvP over the years.)
Grab some popcorn and settle into your chair, as I get on the shaman train and regale you with the joys of playing shaman.
Your power as a shaman goes far beyond an interrupt here and a totem there. When you immerse yourself in shaman gameplay, you discover that shamans alter time and space, speeding up or slowing down battles as you choose, in the spaces you choose.
Elemental shamans pick away at the front lines of battle, flame shock acting as a personal hunter's mark that only astute opponents track. Both the target and their healers must quickly choose between spending a cleanse on clearing this threat (until it reappears a scant 6 seconds later), or waiting to see when the blast arrives. An elemental shaman will cluster opponents together or pull them apart with ice and vines, slowing not just movement, but also positional readjustments of the enemy team. When a second flame shock appears on a different opponent, now an opposing healer can't know which way the burst will land until after it happens. The hit might be a diversion from the focus fire of the shaman's team...or an opener, suddenly shortening the time between a burning blast and a finishing blow.
A good elemental shaman affects not only opponents, but the very field on which they battle. Let other casters focus on single targets. The elemental shaman shuts down chances for enemies, and creates opportunities for their teammates. The enemy stealther, dotted and peeled from a friendly healer. The enemy warrior, slowed and unable to return to support range and suddenly the target of the front line they so eagerly attacked a scant moment ago. The EFC, looking back and seeing their support mired in vines, realizing they must either rejoin their troubled comrades, or risk breaking loose, possibly into the maw of a waiting enemy. Elemental shamans hit when they need to, but shine the most when altering the space and timing between friend and foe, unleashing the true potential of their teammates.
Lower-level enhancement shamans rarely find themselves in Blizzard's balancing good graces (and the current patch is no exception), but when they do...a good enhancement shaman goes from a laugh, to an annoyance, to a concern, to a threat, to a terror. A terror that lingers after the battle is done, making opponents more cautious in the next encounter. A caution that ends up too little, too late. When the dual roiling weapons of an enhancement shaman come for you, it's like a bad dream. The damage keeps building, your legs are slower than they should be, your spells can only hold off the inevitable while your friends remain just out of reach to help. And just when you think a big heal or shield will save the day, a blast hits much stronger than it should have, and ends you. Or worse, the shaman suddenly veers off and uses all of that built-up inevitability to blast into a teammate, and you can do nothing but watch in horror, your abilities spent, knowing your only option is to flee before the shaman's whirling weapons return to finish you. But your legs still don't work, and your comrades recede into the distance.
A good enhancement shaman is a boogeyman. Let other melee classes prey on single targets. The enhancement shaman shapes and cracks battle lines, affecting not just their prey, but those who might help. The enemy healer, interrupted from a distance. The incoming peel, suddenly slowed. The flag carrier, who struggles to stay in a good position relative to their team while unable to shake the burning barnacle until only major cooldowns buy the time the FC needs. But that time serves the shaman at least as much as the shaman's opponents.
Restoration shamans. ...Ahhhh, resto shamans.
Most players want a distinct role. Make me damage incarnate, make me purveyor of immortality, make me an unstoppable monolith with a banner. More often than not, such focused roles see greater success on the battlefield. A warrior may hit you so hard your boots fly off, or wear you down until you fall. A priest may heal big or blast hard. Every class has at least a little utility to help themselves and teammates better succeed, but they typically retain one main focus and players deeply enjoy that clarity.
Restoration shamans thrive on the other end of the spectrum. Of the three specialties, resto shamans alter time and space the most. We speed up battles and shrink distances by slowing enemies, cleansing teammates, and opening on or closing on opponents. Conversely (and more importantly), we extend battles and expand distances with heals that grow ever-stronger when teammates get weaker, peels to keep the front line mobile, and an interrupt to buy more time. With a higher armor class plus a shield, at least two different instant heals and no less than three schools of magic, a restoration shaman can always find a way to contribute. But what does that really feel like on the field?
The horns sound, the gates slam open, and you ride with your team on the front line straight toward the impending clash. You reach midfield and dismount as a warrior flies past you, straight into enemy lines, a teammate's arrows sailing overhead to the warrior's target. The warrior immediately takes a blast, but you snap your fingers twice, and immediately restore half of the damage plus ongoing relief while you shift to the right to get into range of an enemy mage with crackling hands. A quick whisper and another wave of your fingers sees their energies fizzle, and their robes catch fire while two more teammates join the warrior. You step back out of range and hear the telltale descending whoosh of a hidden opponent, and then immediately see the world spin and blades, blades everywhere.
A few seconds later you come to, badly bleeding. A quick whisper stabilizes your health for a second, but you need more. You start a fast spell to revitalize yourself, then pause just before the rogue kicks you square in the breastplate. Not today, you think with a wry smile, and plunge to the ground on all fours as the rogue gets blasted out of his boots by a pillar of light from the sky. Your wolfish grin glances back at a couple of moonkins saluting you as you spring toward the center of your team, and a second later, you launch to both feet with ice crusting the warrior's original target as the warrior lands the finishing blow. A priest's shield is the only thing keeping that warrior alive, and just as the shield evaporates under the arrows of enemy fire, you finish a restorative spell and watch life spring back into your teammate as he yells, and plunges deep into enemy lines, out of reach.
Your comrades surge forward, and you see your standard-bearer take the opportunity to slip away in hunt of the enemy's banner. From a more protected position, you top off random damage on your teammates and slow an enemy monk. You hear the stretch of a bowstring along with the cocking of a rifle, and realize the two hunters to your right set their sights on the mage with the embered robes you hit just a few seconds ago. Anticipating your teammates, you summon a blazing ball of lava. As the hunters' shots land, you spy a shaman on the far end of the field and the mage's robes extinguish. The blaze leaves your hands and speeds toward the mage, you snap your fingers to reignite the mage, causing your molten bolt to double into two, and blast the mage into oblivion.
A horn resounds across the field, announcing the theft of your opponent's banner. With a yell, you surge forward to help push your team's advantage. Suddenly, you find yourself on the front lines. Why did no one push with you? Out of position and a ripe target, the enemy line converges on you and your teammates recede. If you are to die, at least you will stabilize the front line. In a mere three seconds, you slam down a totem and trip up no less than four enemies, shock a fifth, and singe an enemy moonkin in the rare case they wanted to melt into the ether, before a hail of bullets drop you where you stand. In your dying breath, you watch your teammates recede as enemies trample past you. Normally one would claim your insignia, but your teammates keep the pressure up.
You are dead. But not gone. You wait for another twenty seconds or so as the battle rages at a stalemate close to you, occupying your enemies' attention. They push forward again, but you see none of your teammates die. On the far side of the field, you see your standard-bearer standing on a cliff, flag waving brilliantly in the breeze, fighting off the rogue you originally faced. Wait a little longer...a little longer....
There! Something stuns the rogue, and while you don't know what, you see your standard-bearer leap from the ledge to the ground below as you summon the elements to your aid. Your body springs to life not three feet behind from the hunters who downed you, but they're too focused on the front line of your team to notice. A quick restorative whisper brings you half-alive and once again, you plunge to all fours and speed toward an intercept with your flag carrier, when one of the hunters notices you and swivels. You will a sparkling totem into existence at your feet as the first bullets hit. A crackle from the totem singes the hunter, and the hunter spends a second to destroy the totem, buying you the extra time you need to get in range of the waving flag with the rogue in hot pursuit.
One, two, three, in a 270-degree arc, you shock the rogue with a fiery spell to prevent them from disappearing, drop a quick heal on the flag carrier racing across midfield, and materialize a totem next to the throng of enemies on your other side, vines slowing them all as teammates surge to intercept your carrier. Your life quickly drains away as bullets fill your back. Your frost wraps the rogue even as your body falls a second time, and the last thing you see is your flag carrier enter the tunnel toward home while the two hunters trample over you in hot pursuit, followed by the rogue.
A long moment later, you reemerge at the graveyard, horns resounding the success of the standard-bearer, and you're ready to go again.
You don't have be good at what you do to love it, and loving what you do won't necessarily make you good at it. But with some luck and vision, you can make a difference. I'm 'Rise, and
I'm a goddamn resto shammy.
Do you like turning into a FUCKING WOLF SPIRIT !?? Do you like tripping BALLS 
with other farseers , visualizing floating elementals around you
FUCKING LIGHTNING
slanging the elements?! ? WANNA SMACK SOME SHITWITH FUCKING MACES??? USING the power of the ✝ GOD DAMN WIND to hit MULTIPLE TIMES??!
