I can give you a brief informal overview of all of the melee classes.
I'll start with the only pure tank, Lancer. Uses heavy armor and a lance/shield combo. Pretty standard tanking affair. They are very durable, and rely on a blocking mechanic that absorbs all damage and costs MP. They tend to be solid walls of defense in group PvP, and in early 1v1 PvP, rock everyone's socks. Other classes catch up by max level (reportedly). For PvE, they make a standard, simple Tank. Out of the two tanks in the game, they are far easier to play (that doesn't at all undermine their usefulness or the skill of those that play them, as a bad Lancer is still a bad Lancer).
The other tank is a hybrid DPS/tank, the Warrior. Uses light armor (though has a "stance" and associated buffs to get similar defense to the heavy armor wearing Lancer) and dual swords. Fast hits, very mobile. Acts an an avoidance tank when in that role. In PvP, they don't shine until late levels, where they get deadly with lots of stuns and ridiculous mobility. In PvE, their DPS is middle to bottom of the pack, but they come with important debuffs that make them valuable to any group. As far as tanking goes, they are VERY hard to play in a tank role. It's not impossible, and a Warrior can tank all of the content in the game. It just takes a lot more work, and a lot more practice.
Berserker and Slayer are the two pure DPS classes, and are something of an analogue to the two classes mentioned above.
A Berserker, like the Lancer, has a block mechanic to mitigate incoming damage. Uses heavy armor and a massive axe. They are not as mobile, and rely on slow hitting moves with high burst. They have charging skills and are a very durable solo class, in that they are one of the easier classes to solo BAMs with because of their solid hits and ability to pseudo-tank lower end content (not sure how well this scales into higher levels).
The Slayer, like the Warrior, has a dodge mechanic to avoid incoming damage. Uses light armor and a large two-handed sword. Slayers are more bursty than the Warrior, though not quite as fast (and not as slow as the Berserker). I would consider them a pretty standard affair as far as DPS goes, but they come with a solid range attack and a bunch of gap closers. They are considered one of the best PvP classes, if not the best, as Slayers are so well rounded that they have a tool for every situation. They make for a VERY solid, dependable DPS class in general.
Other people can add to/correct the above. These are just my impressions/experiences and the culmination of a lot of reading on the various classes.
Amazing post, thank you ! I'm leaning towards Lancer or Slayer now