So, why can't Melee classes do as much damage as Summoners or Marauders?

I personally think that melees suffer a lost being able to apply their positional skills. It's not impossible, but its very difficult when fighting smart opponents.

Dragoon lost of potencies:

Heavy Thrust: -70 potency, unable to gain Heavier Thrust (+15% damage)
Impulse Drive: -80 potency, unable to initiate combo
Disembowel: -120 potency, unable to inflict -10% piercing resistance status.
Chaos Thrust: -100 potency, unable to inflict damage over time (20 x 30 = 200 total dot potency loss)

Judgement: Without the effect of Impulse Rush, Dragoons lose 25% damage just from the lack of the buff and debuff. They also cant use the massively powerful Chaos Thrust. The end result is just running around the arena using Full Thrust and Jump.

Monk lost of potencies:

Bootshine: Unable to gain the 100% chance of critical damage from behind.
True Strike: -40 potency
Snap Punch: -40 potency
Twin Snakes: -40 potency
Demolish: -40 potency
Dragon Kick: -50 potency

Judgement: Monk's entire rotation gets an average of an extra 40 potency when attacking behind or flank of their target. This, combined with the fact that monks can lose greased lightning when they arent finishing their combos due to CC, Heavy, Sprint, TP, ect. it results as a major loss of dps.

Ofcourse these classes arent completely useless though if we take into account:

Dragoon's assets:

Impulse Rush seems to be the (imo, poor) response of devs knowing about the existence of positional skills in PvP, so you can get the bonuses from a specific combo once. Just once.

Dragoons are capable of dealing out some quick unexpected burst DPS with the use of various off-GCD attacks including:

Power Surge + Jump = 300 potency off-gcd
Dragonfire Dive = 250 potency off-gcd
Spineshatter Dive = 170 potency off-gcd + stun (although in pugs its not a good idea to stun a focused target as a dps)
Impulse Rush = 200 potency off-gcd

A combination of these and Blood for Blood starting from a power-surged Full Thrust could be devestating.

Monk's assets:

Axe Kick is an very powerful and useful PvP skill for Monk, as its potency, Pacification effect, and just the fact that its AoE makes it an essential tool for any Monks in PvP. It has many uses such as an included piece of burst dps, pacifying enemy melee and tank when your team has the same members alive, used to get greased lightning back, or the best use of it would be a combination of those reasons.

Monks have One Ilm Punch which removes beneficial statuses. Its not much of a DPS skill but it is helpful when chasing down healers who use powerful defense and healing buffs that can negate damage as long as it exists. They also have Arm of the Destroyer, a silencing ability, which can prove to be helpful when trying to negate casts on mages that pop Equanimity.

I also have to include Fists of Wind which is very helpful when being kited by a ranged dps or healer.

But why is it that Monks and Dragoons are constantly outdamaged by Summoners and Marauders?

Well first off with the Marauder:

1 - They don't have positional skills.

2 - Marauders don't have to use a lot of attacks to gain buffs; most of their buffs are off-gcd. Maim and Storm's Eye gives buffs, but excecution the said skills doesnt take as long as Monk's Dragon Kick or Dragoon's Chaos Disembowel (without Impulse Rush).

3 - Players usually play as a Marauder and not a Warrior so they can gain additional buffs from other classes. like Blood for Blood (+10%), Raging Strikes (+20%), Internal Release (+crit), Straight Shot (+crit)

When you think about it, Marauders can push their damage to 230% when using:
>Berserk = 50% buff
>Blood for Blood = 10% buff
>Raging Strikes = 20% buff
>Maim = 20% buff
>Storm's Eye = 10% debuff
>Full Swing = 20% debuff

Generally, a Marauder's damage without buffs is lower than a melee's damage without buffs. However, we can just assume that Berserk and Maim alone makes up for that. Then we include everything else. Melees should be bursting when the tank uses Full Swing, but any smart team would have the melee CC'd or heavied (if the tank isn't) to reduce the damage taken on their focus. As a result, melee's can never reach the amount of burst DPS that Marauders can dish out.

And now with the Summoner

1 - Summoners deal most of their damage from DoTs. Because of this, their DoT attacks are significantly more powerful then other classes.

2 - Most of the DoTs that Summoners cast are instant casts. This allows them to freely disintegrate enemies with DoTs unchecked. Additionally, powerful off-gcd skills like Fester and Bane are skills that work after summoners attack their targets, so it is a lot harder to 'waste' a cooldown compared to Axe Kick and Blood for Blood.

3 - Pets are too often undermined and can also gain advantages from independent and dependent off-GCD skills. I see a lot of CC classes checking Scholar's fairies but leaving Garuda-egi fly about. In-fact, I'm sure that pets are about 30%-40% of the summoner's damage.

Summoners don't burst like Marauders do, but they are unarguably the #1 highest sustained dps in PvP. Constantly reapplying, festering, and spreading detrimental dots and debuffs to your party. Since DoTs should ideally be recasted when their time is up, Summoners can whack a bunch of people with DoTs and not lose any dps on their focus. Melee classes can't do that. And unlike Bards / Archers, they don't have a damage penalty for attack from long range. Instantly applied DoTs without penalty or interrupts. Pets that have strong cooldowns and often ignored. Not much more to say.

I think that Melee classes could use a PvP buff. At this point, if frontlines doesn't have imposed parties, melees would get the bad end of the stick because no one would want them.