I figured that since Neverwinter is an action based MMO, it would be good to compile a list of tips and tricks that may help some people survive their next encounter, or save our veteran players from giving the same lessons day in and day out. Some will be fairly common knowledge, even mentioned in-game, but you would be surprised how many people don’t know about them.

If you know any tips that I may have overlooked and think they deserve to be in this thread, or I am wrong about something, feel free to post and I will add/change them if necessary. I will also add more tips and edit certain ones already posted as I find them during Open Beta.

Combat Advantage / Positioning
Combat advantage is a major part of the combat of Neverwinter, and boosts your damage quite significantly (If someone knows the exact % damage increase it gives, that would be appreciated). The easiest way to gain combat advantage against a target is to flank it from opposite sides.
If a fellow player, or even a companion is attacking something, a small white quartercircle will appear at the feet of the enemy (the white area may increase if multiple allies are attacking it at different angles). If you stand on that side of the enemy, the quartercircle will turn purple, and all damage you do while standing there will be increased, and your damage should have a sword icon next to it to confirm.

There are other ways to gain combat advantage, such as hitting the opponent with Dazing Strike as a Trickster Rogue, which will make all damage dealt into combat advantage damage for a limited time.
Certain feats will also give you combat advantage under specific circumstances, such as the Control Wizard’s Renegade Paragon feat, ‘Nightmare Wizardry’.
As a striker, try to position yourself so your defender gets combat advantage off of you, as it will help increase your damage, as well as helping them hold aggro with increased damage for themselves.
Enemies can also receive combat advantage for attacking you from multiple sides. Try to avoid being surrounded by using your evasive skill (shift + wasd) to keep all enemies in front of you.
Whenever a red area appears on the ground, do your best to get out of it, or block it as a Guardian Fighter.
When playing in a group with multiple melee players, if you want to be directly healed by the cleric, try stepping out of the fight a small bit and into the cleric’s line of sight. This will make it infinitely easier for the cleric to target you for heals. Otherwise, continue attacking the enemies, as they should have Astral Seal on them, as well as other healing effects from the Cleric.
Let the Defender of your party go into a mob group first. This will allow them to maintain aggro better.

Know Your Role
Neverwinter is an MMO where classes have clearly defined jobs that they are expected to do in a party. If one person slacks off, it can make for a much tougher encounter, or even a wipe during a boss. I can’t tell you how many times I’ve seen situations where everyone is just trying to DD, instead of doing what they were made to do. There are four roles in Neverwinter, and every class falls into one. They are Striker, Defender, Leader, and Controller.

Striker: specialize in dealing high amounts of damage to a single target at a time. They have the most concentrated offense of any character in the game. Strikers rely on superior mobility, trickery, or magic to move around tough foes and single out the enemy they want to attack.
Defender: have the highest defenses in the game and good close-up offense. They are the party’s front-line combatants; wherever they’re standing, that’s where the action is. Defenders have abilities and powers that make it difficult for enemies to move past them or to ignore them in battle.
Leader: inspire, heal, and aid the other characters in an adventuring group. Leaders have good defenses, but their strength lies in powers that protect their companions and target specific foes for the party to concentrate on.
Controller: deal with large numbers of enemies at the same time. They favor offense over defense, using powers that deal damage to multiple foes at once, as well as subtler powers that weaken, confuse, or delay their foes.

Classes:
Guardian Fighter: Defender
Great Weapon Fighter: Defender/Controller
Trickster Rogue: Striker
Devoted Cleric: Leader
Control Wizard: Controller

Understand your Powers
There are several different types of powers in Neverwinter, and there is a time and place to use them. While you may be able to get through solo content just spamming cooldowns, it’s generally not a good idea in group content.

Your “Daily” Powers (the skills mapped to 1 and 2) are not 1 use per day. Generally, with a build that isn’t necessarily focused on Action Point Regen, you can use one once every few encounters.
Several encounter powers (the skills mapped to Q, E, and R) have important effects, such as the Guardian Fighter’s Lunging Strike allowing him to close the gap between himself and the enemy very quickly. These shouldn’t always be spammed for increased DPS, as you might need the effect before it is cooled down. However, that is not to say that they should NEVER be used for pure DPS. If you know you won’t need it again before the cooldown, the increased damage they do is always helpful.
Control effects, such as stuns, dazes, freezes, knockbacks, etc. are resisted by certain larger mobs and bosses.
Control effects can interrupt skills from enemies (including those that create red zones), except against enemies that are immune to the effects.
When in a group, try not to use knockback powers on enemies that melee classes have a handle on. This ends up making the encounter take longer by lowering the overall DPS. Knockbacks can still be useful to get something off the cleric, or knock an enemy into a bottomless pit, but in general, be wary of when you’re using them.

Communication
In group play, communication is a VERY important thing. Even in PUGs, try not to be antisocial just because you don’t know the people you’re playing with. Everyone is there for a common interest, work with it.

If you have a headset+microphone for your computer, that is the best option for easy communication between allies. The game has a built-in voice chat, use it.
Don’t assume everyone can read your mind. If you’re planning on doing something specific in a certain area, let the team know ahead of time.
If you’re in a PUG, ask at the start of the instance if anyone is new to it. It will save a lot of headaches later on if you can explain something to them early, rather than assuming everyone knows how a mechanic works and being frustrated that they don’t.

General Combat
Basically anything else that doesn’t have a defined ‘genre’ during combat/grouping will go here.

Use line of sight to your advantage against enemies. If there are ranged mobs, have one of your party members grab their attention with a ranged power and run behind a wall or pillar. This will cause the ranged enemies to follow the person around the corner, which will put them in a nice cluster for everyone else to AoE down.
As a ranged class, use high ground to your advantage. If you can manage to position yourself on top of a tall building or wall near an encounter, most melee enemies will not be able to reach you. However, even if they can’t, stay out of their red splats, as you can still be hurt by those skills.
Healers in Neverwinter won’t be able to keep you alive if you’re face-tanking. Try to avoid as much damage as possible through dodging/blocking. Or if you must face-tank, use potions to offset the difference in damage vs heals.
It’s generally frowned upon to roll “Need” on every green/blue drop, and is a good way to get on people’s ignore lists for further dungeons. As a courtesy rule, roll “Need” only on items that are colored/gray (ones your class can use/eventually use), and “Greed” or “Pass” on anything red (items your class will never be able to use).
Don’t rush into combat expecting a Cleric to have your back at all times (even if it’s your Companion). You’ve got potions, use them when you need them, as waiting for a heal will sometimes result in death.
You will occasionally run into enemies that summon minions. Try to keep these guys incapacitated as long as possible. If they do successfully summon their minions, continue to focus on them, as focusing on the minions is just going to allow the main enemy to summon more.

I look forward to getting feedback from people, and would be much appreciate it if the Hero pack purchasers could post additional things they discover from their early access gameplay.

References
The “thank you” section to those who helped evolve this thread by adding additional information either directly or indirectly.