First off, there's the extended water totem questline. Both Horde and Alliance shamans get sent all over the place restoring the balance of the elements and are eventually rewarded with their water totem. I was level 23 on my horde shaman and 22 on my alliance by the time I finished all of the quest chain. We've already covered what you get at level 20, so we'll focus on level 22 now. My personal favorite is Cure Disease, but Water Breathing makes a lot of STV and Desolace quests easier and there's nothing wrong with the Poison Cleansing Totem.
Level 24 is a little anemic with the Frost Resistance Totem (maybe we'll get more use from it in Northrend, but at the time I don't think I ever dropped it) but 26 picks back up again with Far Sight (a fun ability you can use to scout around with or to annoy Hunters using Eagle Eye on you), Magma Totem (a nice AoE pulse fire damage totem) and Mana Spring Totem (nice if you're an elemental or resto shaman or intend to be). 28 is also a decent level for abilities, providing you with Fire Resistance Totem (its heyday is gone, but who knows when a fire dungeon may come again? It's still nice on FR fights), Flametongue Totem and Water Walking, one of those abilities you will make use of surprisingly often. I can still remember using it to go to the surface in Feralas and eat and drink on the water, making finishing quests underwater much easier. It's also faster to run on the surface than swim, if that's ever an issue. I realize Water Walking isn't really a make or break shaman ability, but I've always enjoyed it.
30 is where Shamans begin to really rip the roof off, with stellar abilities like Astral Recall (which you can't cast in combat, why do I always forget that I have a hearthstone too), Reincarnation, which you will use to irritate people in PvP and to help either prevent a wipe or help speed recovery from a wipe in various instance, Grounding Totem, Nature Resistance Totem and (finally) Windfury Weapon. Believe me, you will notice a huge difference in your performance in melee (especially if you're specced heavily in Enhancement talents at this point) once you get Windfury. And there's nothing wrong with being able to hearthstone every fifteen minutes, which is what Astral Recall effectively does. Grounding Totem combines nicely with old standby Earth Shock to help you avoid getting fried by casters, and Nature Resistance Totem... will be dropped a lot in AQ if you ever end up there for some reason.
Heck, they can't all be winners. You get four awesome abilities at 30, cut the NR totem some slack. It tries its best!
Level 32 shines some love on the aspiring elementals among us with Chain Lightning, and gives rogues, warriors and retribution paladins a reason to love you with Windfury Totem. Feral druids won't care, as WFT doesn't do anything for them. Ah well. Can't please everyone. Level 34 brings the awesome, senses shattering might of Sentry Totem. Do not look directly at the Sentry Totem. It can see into your very soul. You may very well drop this totem in a Warsong Gulch sometime, it's just that powerful. Seriously, half the reason raids take shamans at all is that they covet the titanic power of Sentry Totem. Don't listen to their pleas: only drop this totem as a last resort, as it may well make warlocks and mages obsolete by the sheer fury and majesty of its planet splintering power. Quake, mortals, and feel the iron grasp of despair upon you, for Sentry Totem may be unleashed by a shaman at any time! I will be able to watch from some distance away as you one shot my totem!
I may be overselling Sentry Totem just a bit. It's possible some other shaman out there has actually found a use for it. Level 36 introduces you to the moderately more useful Windwall Totem, which does for ranged attacks what Stoneskin does for melee, and level 38 grants you Disease Cleansing Totem, which is one of those totems that, like Poison Cleansing Totem, you'll use sparingly but love having when you need it. Finally, at level 40 you can train to wear mail (finally, all that Scarlet Monastery gear you spent levels grinding for can be equipped) and you get the really rather awesome Chain Heal, one of the most interesting heals in the game. You'll make more and more use of it as you level.
I don't personally like to tell people what spec to pick... it's arrogant of me to pretend to know what you are good at and where your interests like... but to my mind, it's very hard at this level to build a nice elemental set without wearing a lot of cloth, and if I wanted to be wearing cloth I'd have rolled some other class. I have always leveled enhancement (by 40 you can have Dual Wield or Stormstrike, and whichever one you don't get you can get at 41) and I've never regretted it. Elemental DPS, to my admittedly limited eyes, really takes off in the Outland when you can start getting nice spell damage mail. Resto is a solid spec, but I don't like leveling in it for the same reason I don't like levelling as a pure Holy paladin or a Protection warrior. This is a matter of personal taste and opinion, but I believe enhancement to be the easiest spec to gear for and level in at these levels, as there's lots of leather out there with the stats you'd want. If you do choose to level enhancement, the 30's most reward a big, slow 2h weapon so as to hopefully avoid the Windfury Weapon cooldown. Once you can DW at 40 (assuming you choose to) you'll want two slow weapons if possible. It can be hard to arrange this, so don't feel bad if you're using a faster than ideal weapon at this level, it's not easy to arrange until itemization catches up with you in Outland.
Starting out you can get the Arced War Axe in Shadowfang Keep, the newly upgraded Reef Axe from Blackfathom Deeps, the excellent Corpsemaker in Razorfen Kraul serving as a really good replacement for them. Alliance shamans can get Bonebiter at a surprisingly low level if they have a few higher level friends willing to run SM with them and it can last a solid ten levels. Thermaplugg's Left Arm is an improvement on Corpsemaker as well, as is the Obsidian Cleaver if you're willing to check the AH. Manslayer and the Hellslayer Battle Axe can also serve in this capacity, and if you're combing Scarlet Monastery anyway you might as well get Ravager off of Herod, it lacks stats but is still a solid axe. Heck, just take everything Herod drops, you'll wear it all once you hit 40.
For maces, the Slaghammer and Dense Triangle Mace are very nice AH blues, upgrades from the weapons you might be wielding from the pre-20's instances like Smite's Mighty Hammer or Rhakh'Zor's Hammer, both from Deadmines. In fact, most of the maces you can pick up at this level are Bind on Equip - only the Manual Crowd Pummeler from Gnomeregan and Mograine's Might from Scarlet Monastery are BoP in this level range. Even if you focus more on elemental or restoration talents as you level, I suggest getting a decent 2h for when mobs get up in your face and a few windfury hits are in order.
The new quest hub in Dustwallow has some nice starter mail if you're not lucky in Scarlet Monastery, if you're looking to avoid the ganking in STV (although I'm sure there's plenty of PvP there now, at least with Desolace, Dustwallow and STV you've got decent options of where to quest in the 30's thanks to 2.3) and I recommend taking a look there. Don't forget instancing, as many do, as it's a good way to figure out your class even if it's not strictly speaking necessary to level, and the better gear is still to be found there. You'll start out running Wailing Caverns and Shadowfang Keep as you hit 20. Drops have been improved in 2.3 and the instance levels tightened up. You'll move on to Blackfathom Deeps then Gnomeregan and Razorfen Kraul. Between 30 and 40 you will farm Scarlet Monastery the most (Herod, baby, Herod... and Mograine only slightly less) and then Razorfen Downs and even Uldaman. These instances, combined with outdoor questing, should carry you to 40 in short order with the new leveling schemes.
Okay, that's enough for today. Next time, either 41 - 58 or a more detailed breakdown of gear for 21 - 40 for each aspiring spec. Or an entire post about Sentry Totem. I'll do it. I'm crazy.