Power Pools
Power pools are sets of powers (four in each set) that are accessible to all ATs starting at level 6. They increase the variety of builds that players can take which is desperately needed because you can get kind of depressed putting a lot of effort into a character only to find out that everyone else built theirs the exact same way.
Each pool works the same way: The first two powers are available from level 6, the second at 14 (and you must have taken one of the two level 6 powers), the last at level 20 (and you must have taken any two of the other three powers).
Fitness
Being able to play without a lot of down time between battles is the primary reason I recommend this pool. Health increases your heal rate and Stamina increases your endurance recovery rate. Together, you won't have to rest between most battles and it's less likely that you'll run out of endurance during a fight.
The only question then is whether to take Swift or Hurdle as your prerequisite power. If you take Super Speed for a travel power, choose Hurdle. If you take Super Jump, take Swift. If you take fly or teleport, either is probably fine. Some people take Swift AND Hurdle and slot them to max to use them as a mini-travel power instead of the classic four (TP, SS, SJ, and Fly), but there are other reasons to take standard travel powers which I will explain below.
So to summarize:
Swift
If you prefer to run faster and don't want to take Super Speed, Swift is a good choice. With the recent change that makes fly, hover, etc faster if you have swift, I expect this to become more common versus hurdle for that reason alone.
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Hurdle
If you like jumping higher or if you take Super Speed, Hurdle is the better option.
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Health
Since there's really no point in taking both Swift and Hurdle when you take a travel power, you might as well take Health as your second fitness pool power. It will also
increase your health regeneration rate and reduce between-battle downtime.
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Stamina
It's the reason you took this pool. Make sure to take it as soon as you can and fully slot it with Endurance Modification Enhancments.
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There are many places online that will say that building a character that doesn't take Stamina is basically impossible. A few notes about that: First some characters really don't need it because they have powers that give you endurance (ex. Regen powerset, Warshades because of Stygian Circle). Also, whenever I was teaming with my Defender, it just wasn't necessary because my defensive powers didn't take enough endurance to cause problems (when teaming I would often leave the bulk of attacks to other players).
Speed
Flurry
You smack an opponent repeatedly with a punching combo. Does a decent amount of damage and has a chance to disorient, but has a fairly long activation time (about 3 seconds) during which you won't be able to launch other powers. Take it only if you need an extra attack.
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Note for scrappers: The critical hit is calculated off each hit you make on an opponent. Since flurry is a string of hits, each one should have a chance to be critical. Flurry may be a good option for this reason alone.
Hasten
Many ATs have powers that take a long time to recharge. Having Hasten and firing it off pretty much every time it comes up can keep your slow recharging powers available at the start of most battles.
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Slot with the maximum of three recharge reducers so you can use it right away. If you're playing a character with a lot of long-recharge powers, just fire Hasten every time it comes up rather than wait till you're about to fight.
Super Speed
 I'm a blur with my stealth power an SS together |
 In the air, with SS on, I'm a speeding comet still :D |
I took this because the stealth abilities of Super Speed and any other stealth power stack to give you full invisibility. With very few exceptions, you could in the middle of a group of enemies and make faces or slap them (/e slap in the chat window) and they won't do anything to you (as long as you don't activate any powers that affect enemies).
This gives you the extremely desirable ability to sneak through missions that only require you to find glowies or defeat the mission boss and enemies around him. This is
especially useful for cutting down the time it takes to complete TFs (though you'll need Recall Friend to really make that work). There are also a wealth of cool
screen captures you can take that you could never otherwise.
Don't get me wrong though, mostly the reason I took this power on any of my characters was for speed. Even though Teleport is technically faster, with Super Speed, you can really feel the speed (especially if you took my advice and built a shorter character).
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There's really no reason to add slots to SS. You might put in an Endurance Reducer if you get tired of turning off SS before battles though.
Because Super Speeders can easily get stuck in ravines, or behind large walls, it's often desirable to combine it with Super Jump or Fly powers. The simplest way to do this is to take Hover which is a single power that needs only a few slots to be useful.
Whirlwind
You start spinning and toss enemies into the air. This deals no damage, but keeps people from closing to Melee range. For builds that use AoEs or melee range attacks, this is pretty much a waste of time except possibly as an escape mechanism.
As a support character, you could use it to protect yourself, or run into the crowd and save a dying friend by tossing every enemy near him into the air (though this is likely to draw fire to you). Since you can continue to attack while Whirlwind is on, a blaster would benefit by keeping enemies at a distance while firing at them.
The problem is the enormous endurance cost. It's designed to keep you from leaving it on and battling at the same time.
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I've only taken it on one character (Gale), and it's all kinds of fun 
Since she has a pet tornado, it seemed like a good idea for her to go spinning around too. Not only that, but it's a challenge to find a way to use otherwise abandoned powers.