1. Prophecies is fun and diverse, Nightfall is very awesome with Heroes and other things, Factions is meh.
2. JOIN SoE! Well, SOE. (don't know the difference? Too bad, I'm sure someone will clear it up though

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3. elementalists dish out elemental (fire, water, air, earth; fire is the most used and my favorite, earth is defensive, air is used for quick spikes and speed, and water has a plethora of hexes) damage (most damage in a single hit in the game I think) while having sacrificed armor, warriors soak it up and tank and are generally thorns in your side and can use shouts to boost stats for a few seconds, they use hammers, swords, or axes. Mesmers use sneaky under-handed mind melds to accomplish their own goals (I izzint prejoodissed!!), ritualists summon powerful spirits and... um... I forget. Assassins are the most looked down upon, IMO, and reportedly the hardest to use, but they can have the highest dps (damage per second) of any class... then there's monks, which heal and protect or use smiting prayers to dish it out. Necromancers are the Hot Topic-frequenting gothic nerds you see sulking around bingo parlors in their off-time, but they can curse and sap health away and have undead minions... rangers use bows and ranged attacks and can have a pet, and also lay traps and can summon spirits (not so powerful as the ritualists', mind you). Paragons? They're like angelic leader people who use chants, which are like shouts but last longer sometimes aaaannnddd I forget the rest. They throw spears, if that's kewl to you. Dervishes wear sissy robe-dress things but have awesome scythe weapons that can hit up to three enemies at once (if you have Apply Poison, which causes your physical attacks to inflict poisoning, it's really awesome) and they can even shapeshift, sort of, with certain elite skills.
That's my totally unbiased opinion on the classes...