I agree with everything Velouce said. I play a Warrior, so lets compare this class with a Sorcerer:
Rage Strike vs. Varetyr Spear:
Assume I can have a Rage Strike that requires no build-up points (Rage points) to use and that deals double damage to stunned targets.
I would say: "Deal!"
Pommel Attack vs. Frost Driver:
Assume a Pommel Attack that stuns a target for 5sec instead of 3sec and has a cooldown of 5sec. After an successful stun attack, the target gains a short resistence against further stunning attacks.
I would say: "DEAL, DEAL, DEAL!!!"
The issue here is that RO2 developers forgo the normal "restriction through cooldown" path and chosen a "restriction through minor effect" alternative. No other class does this. Likewise, Frost Driver is the only effect/debuff that causes an additional effect. Just assume it would be the same with stun, silenced/disoriented status effects as well? Other games like Dragon Age 2 did this, but it's simply unfair in RO2 to give this to a single class.
Battle Tactics vs. Wind Arms trigger:
What Wind Arm really does: 30% chance to trigger twice for double damage. So on the average, you could cast 3.3 Ice spells to cast 2 Lightning spells with double damage. 2 out of 5.3 (3.3+2) means a 37.5% chance for double damage. On contrast, you need 250 INT to deal +100% damage on crits with Battle Tactics.
I would also definetly say: "DEAL!".
A 37.5% crit chance is really nice to have, especially if it's IN ADDITION to a normal crit chance. Just imagine, if an assassin would have a similar skill that enables him to get into a "shadowy state" where he is still visible (but blurred) that gives him the chance that the next two attacks would do critical damage. They would love it. I would definitely forgo Battle Tactics for a Wind Arms trigger, just because you can calculate when you want to crit (the Wind Arm trigger stays active for a while).
BTW: This effect isn't even balanced by itself. If you use it with Lightning Bolt, the benefit is much, much lower than with a Spear or Thunder.
HP vs Healing:
This is the most common argument made by sorcerers: "Sorcs aren't overpowered, because the do not have the same HP total as a tank".
Basically, this is nonsense. Let's start with a simplier example: Renovatio. Although a Priest doesn't have the same HP total as a tank, it's harder to kill one just because of Renovatio (and Highness Heal, Coluseo Heal, Sanctuary). If you attack a Priest, you have to add the amount of damage healed over time to calculate the damage you have to deal in total to kill that priest.
Let's assume that Renovatio heals 500 damage per 2sec trigger, whereas you deal 1000 points of damage during the same period. If that priest has 8000 HP, how long does it takes to get that priest down to 0 HP? Without healing, it takes 8x 2sec., With healing, it takes ~16sec. During this time, you dealt a total of 16000 damage. In other words, in this example, Renovatio alone was worth a 100% HP increase.
That example might not be accurate, but it shows the benefit of heal.
The sorc uses the same strategy, but in a slightly different way:
Melee vs. Ranged Attacks:
This is the biggest issue, because its hard to balance.
Generally spoken, its impossible for my Warrior to kill any Assassin, Rogue, Wizard, Priest, Sorcerer, Crecentia, Soulmaker, Monk or Knight. All these classes have a ranged attack. Due to the fact that my Warrior isn't faster than any other character, its not possible to get into melee range at all. They can simply run away. It might take long for a rogue to kill me in this way, but sooner or later, his poisoned throwing dagger would kill me.
Therefore the only chance a Warrior has is to either kill mobs (they are too dumb to kite) or take opponents by surprise as they is busy killing each other. Other melee classes have similar problems. A rogue or assassin can stay invisible, so it's hard to run away.
You would now expect that this major drawback is balanced out by some other facts... f.e. that all classes with a ranged attack must stay put to attack ... so that a melee class would get closer on every attack.... or you would expect that the melee damage is much higher, so that once I get into melee the drawback is inversed. This would make melee a high-risk, high-reward playstyle. But RO2 developers chosen to do the ridiculous: They gave most ranged classes an additional slowing or immobilizing attack and an equally or higher damage output.
Well there is some kind of hope: Charge, Leap or teleport attacks. With them, you can overcome the distance between you and your target, but it turns out that this is irrelevant most of the time, if you can't keep them at melee range.... so this is where your damage output comes into account and the fact that your target can slow you down or immobilize you and get into ranged mode again.
To sum it up:
A Sorcerer has a higher overall damage output, has a higher burst damage, higher attack range, better cooldown/build-up skill and more HP over the course of time.
Why it isn't this a ko-fact?:
Beside the already mentioned hope for melee characters based on the chaotic business in the Coloseum, one the most popular whining arguments anti-sorc-nerf is ganging up against sorcs. Well, although all classes suffer from the simple fact that being attacked by 2 players equals receiving twice the damage, it can balance out overpowered classes a bit. However, this can become a necessity, a must-do burden for players to negate an existing inbalance.
Another sort of balancing can be similar to the "Scissor-Stone-Paper-Lizard-Spock" game. If a ranger can kill tanks easily, but are killed by sorcs, whereas the latter is easily killed by rogues, who are killed often by tanks. This is a good way to assure balance, if there is a way that the numbers can increase/decrease accordingly. But RO2 is a bad choice, since the absence of one part can destroy the whole balancing concept.
I sometimes play my friend's warrior class in PVP for him. Yesterday i managed 2nd place in 4th round with 1 player kill. I killed 13mobs and managed 2nd place. I standby everything i said. Sorcs don't need any changes for them. If you are a smart skilled player you can avoid their damage, unless you are a ranger. Also, when playing as my friend's warrior, he randomly gets 2 kills per rage strike, so I don't see why it should get a buff when he can pull 2 kills for his 1 attack. I really think you need to spend more time getting better instead of attacking other classes who struggle just as much as you do.