What Zombolini posted was an opinion. One which I disagree with on several points. Maybe I'm right, maybe Zombolini is right, maybe we're both wrong. I don't know so this is a learning opportunity and why I'm bothering to respond.
Warriors - Physical DPS class, with the exception of Warr/Mage which is a magic DPS class (still melee mostly though). Warr/Mage is the "flavor of the month" - probably the highest DPS class there is thanks to recent changes. Could be nerfed back soon, who knows. Warr/Knight is a semi-tank type, but most Warr/* combos are just pure physical DPS.
Mages - What Zombolini said, but add that they specialize in short burst damage. Also some of the best area effects.
Priests - Zombolini is mostly right, but I'd put them as equal healers to druids and not better. Priest/Scout is kind of an exception as that is usually built as PvP focused damage dealer.
Knight - Human only. Yes, hard to kill and very competent instance tank, but Warden tank or Champion tank are almost as hard to kill and can deal MUCH higher damage. Personally, I wouldn't want to be a Knight right now.
Scout - What Zombolini said. These were top DPS a couple chapters back and have since fallen out of favor. Good for anti-rogue in PvP.
Rogue - I hadn't heard about major DPS reduction when energy runs out. Sure it goes down, but by a lot? (If you're actually a good rogue player.) Comparing Rogue to Mage, I commonly see them roughly equal on short burst fights. But as soon as the fight goes 3-5 mins or more, then the rogue and other physical damage dealers do way more damage.
Warden - Warden/Warrior is the tank combo. Others more damage oriented. There have been substantial recent changes to Warden DPS and I'm not entirely certain how they do now. Probably competent, but not very top.
Druid - Elven healer. Again I'll disagree with Zombolini and say it equals priest. Different, but competent in every situation I can think of.
Champion - Champ/Rogue I play, so maybe biased. It can switch between DPS and tank roles. I've mostly seen folks focusing on the tank part. As a tank I regularly out damage mages on long boss fights. Also often get the top kill score in PvP thanks to having both the attack power of a DPS and the defense of a tank. Champ/Priest is another combination that I respect - as a lower DPS instance tank. But other Champion/* combinations don't seem as good.
Warlock - Champ and Warlock were released relatively recently. Agree with Zombolini that I haven't yet seen anyone make a "good" warlock. Play 'em as alts, have fun, but don't expect them to do particularly well at the high end.