The 10's are always levels with lots of new stuff. Level 20 is no different, and really, is much more exciting than level 80, where you'll only pick up one new skill, and lots of new (expensive) ranks.
Skills, Level 20
- 20: Aimed Shot (Rank 1), Arcane Shot (Rank 3), Aspect of the Viper, Disengage, Freezing Trap (Rank 1), Mend Pet (Rank 2)
In Vanilla WoW, and through most of the Burning Crusade, Aimed Shot was a 3-second cast and hit much harder; at one time it reset your Auto Shot timer, as well. "Clipping" Auto Shots used to be a DPS concern - losing Auto Shots to your special shots meant lower overall DPS. There was a golden period for Night Elves where it didn't break Shadowmeld until it went off, allowing Night Elf hunters to have serious ambushing capabilities in PVP. (Having played a Night Elf through that period, I will confirm that it was awesome.)
Alas, much of that is no more, and Aimed Shot is now an instant cast, 10-second cooldown, moderatly hard-hitting, Mortal Strike type ability. It's quite useful in PVP (that was much of the impetus of its change to instant Mortal Strike), and with it being an instant cast, it earns a place in the hunter shot rotation again, which it hasn't had since the days of Molten Core, save as an opener. Since you don't have Steady Shot yet, the rotation is fairly simple - just hitting Aimed and Arcane Shots whenever they're available. You're still beginning with Serpent Sting and possibly Hunter's Mark.
You may be noticing your pet not holding aggro very well, especially in the upper parts of a 0-9 range of levels. Pets learn a new rank of Growl on the 10's (automatically - pets are very low-maintenance now), so they're not pushing out as much threat as you in the upper ranges of the brackets. So when your pet dings 20 (you'll notice that your pet has its own experience bar, in his tab in your character sheet), he'll pick up a new rank of Growl and start holding on to things better again. He'll also pick up his first talent point, and get a new one on levels divisible by 4 (24, 28, etc.).
So! Besides Aimed Shot and pet skills and talents, you're going to pick up new ranks of Arcane Shot and Mend Pet, and then you have three new skills. You pick up an Aspect at level 20, the Viper. Viper is your mana regeneration skill. It used to be learned in the 60s, as it was a Burning Crusade addition to hunters; at that time, though it was a passive regeneration boost, and I basically raided exclusively in Viper. Now, though, it's a mix of active and passive, and you deal 50% damage while it's active. Basically, turn it on during down time, or when you run out, but keep Hawk up in combat otherwise.
Besides the new Aspect, you pick up Disengage, another hunter skill that has been completely redesigned since the days of Vanilla WoW. This used to be similar to the rogue's Feint - a melee attack that reduced your threat. However, it also stopped your attacks completely. It was useful in PVE - if your Feign was on cooldown, you could hit it once, maybe twice, and your pet could usually recover aggro. It was useless in PVP, though. So now, instead of reducing your threat, it flings you backwards through the air for up to thirteen yards. It is sensitive to the slope of the ground - you can't really Disengage uphill. Disengaging downhill can lead to flights much farther than thirteen yards (and falling damage). And you can disengage off cliffs, or into additional mobs. (Into lakes of fires. Ahem. I did miss the lake. But not the extra elementals...) If you're snared, you won't go anywhere.
The last new thing you get at level 20 is Freezing Trap. Freezing Trap is another ability that will, basically, save your life. If you pull multiples and you and your pet can't handle them, drop a Freezing Trap, and it'll be frozen, in a block of ice, for about 10 seconds. As your ranks increase, your Freezing Traps will last up to 20 seconds, and with the 30 second cooldown, you'll be able to chain your traps. But not yet. Traps have a 1 second arming time, so it's always good to drop the trap before you'll need it - if you're looking to pull a group, for example. It's handy in PVP as well, although its duration there is capped to 8 seconds, as most crowd-control abilities are. Combined with Disengage, which you also just picked up, Freezing Trap will be part of your arsenal in PVP to get at range.
Starting in patch 3.2, you can also learn to ride and buy your first mount. The cost is a far cry from the 100g or so it would cost you at level 40; now it'll be closer to... 4g. I would be jealous in a get-off-my-lawn kind of way if it didn't also mean my bank/auction toon will be getting a mount.