The Art of Warcraft: Positioning in Arena

"The terrain is to be assessed in terms of distance, difficulty or ease of travel, dimension, and safety."

--Sun Tzu

When it comes to being new to arenas, knowing how a particular map works will help you strategize around it, in terms of LOS, avoiding CC, and knowing where your positioning is the strongest and when it is the weakest.

Distance 

The distance between your entrance doors can determine how the beginning of your match will play out.

This is quite true in the following circumstances:

Warlock Portals: Many warlocks like to summon a portal in the beginning. Usually, this can prove to be frustrating for melee when they are about to close in for a kill--the warlock teleports to the portal, and the melee is forced to either switch targets or chase the warlock (usually not a good idea if the distance is too big--you could end up running into the enemy and put your own teammates at risk as they have to move out into the open to heal or protect you).

So, what does this have to do with the distance of between your entrances? In certain arena maps, it IS possible to stop the portal summon, given the right circumstances. Usually, having a teammate with a ranged silence or stun is useful--hunters and DKs for example.

• Blade's Edge Arena - The best portals are cast from one side of the bridge to the other. These can be stopped if you go straight for their side of the bridge at the beginning (instead of going through the bridge).
  It's harder to stop a portal if they cast it on the bottom or closer to their entrance.
• Dalaran Arena - Usually possible to stop the portal here. 
• Nagrand Arena - Possible, if you go by mount. However, the entrance is so far that the teammates have a better chance of stopping you before you make it in time to silence/stun the warlock. 
• Ruins of Lordaeron - These are nearly impossible to stop, unless the warlock is slow.
• Tiger's Peak - The entrances here are relatively far from each other. If the warlock stays behind the pillar and casts the portal sideways, it can be hard to stop. Otherwise, it is possible if one of your teammates has a speed increase or if you go in by mount. 
• Tol'viron Arena - See Nagrand Arena. 

Difficulty 

Some maps are better suited to certain teams. For example, due to the small amount of space on the bridge, Blade's Edge Arena can be great for classes with AoE snare effects, such as the shaman's Earthbind Totem, a mage's Frost Nova, and a priest's Psychic Scream. Some melee classes may be weaker on the bridge because they're in range for a variety of area CC.

Bridge in Blade's Edge Arena

• Multiple levels in the Blade's Edge Arena can create LoSing problems for the healer.
• Teammates maay be pushed off the bridge or feared onto the platform out of LoS.
• Staying on the bridge or the platform doesn't provide the healer with any means of avoiding CC from the enemy. There are the skinny little pillars at the ends of the bridge but those are tricky to work with. The pillars on the bottom would give the healer a better shot at LoSing the enemy.
• Sometimes a team can lure the enemy team into the bottom level, where the supporting pillars of the bridge work as LoS opportunities. But if the enemy team is made of casters, it might be hard to lure them in the beginning since they have no reason to come down to you. They'd only come down if there's a good reason to, such as the high possibility of landing a kill (if you're vulnerable or at low health). 

Ease of Travel

Don't travel so far that your friends cannot catch up to you. If you make it hard for your healer to travel to you, you can potentially lose.

• An example of this was when a warrior hopped onto the bridge in Blade's Edge Arena and Heroic Leaped on top of my hunter buddy, leaving the warrior's own healer behind. In the next three seconds, the hunter's Scatter, Trap, and Explosion Trap threw the healer OFF the bridge. This created an enormous gap between the enemy team and led to their loss. 

Dimensions

Be aware of all dimensions of the arena, where the pillars and entrances are positioned. Here are some examples that you're probably aware of:

• Dalaran Sewers has two levels: platform and the bottom ground. It also has a rush of water in the center that can act as a temporary pillar/LoS in the center. 
• Lordaeron Ruins has a tomb that you can LoS enemy players by running AROUND the tomb. You can also hop on top of the tomb to get to the other side faster. 
• Blade's Edge Arena: High bridge, need I say more?
• Tiger's Peak--two semi-circle raised platforms on the sides.

Safety

Traveling too far to chase your enemy will put you and your healer in danger. You will get out of range or be LoSing your healer. To heal you, your healer has to run out into the open--this leads to them getting CC'd by the enemy.

Being too far from your healer and/or LoSing them will get you killed. Don't go alone, and always keep your healer within range. 

Some players will try to lure you away from your healer. Don't fall for it. If they pull back, you pull back too (or bring your healer). Switch to a target closer to you, like a pet or closer enemy player.