Class Overviews
On Armor: Classes who can use leather can also use cloth. Likewise mail and plate users can also use leather and cloth. It doesn't happen often once you've made it to level 10 or so, but sometimes it does.
On Mounts: Mounts are atainable at level 40. It costs 20 gold to learn to ride, and 80 gold to buy a level 40 mount. When mounted your speed increases 60%. At level 60 you can get an Epic Mount for 1000 gold which increases your speed 100%. Both training costs and the costs of buying your mount are effected by the Honorable faction status discount of 10%, which is good because the prices are massive at those levels. I didn't get my mount until level 47, and that was only because I borrowed 34 gold from RED_SCARE. Most people only train in things they need once they hit level 30 until they get their mount.
On Buffs: Druids, Mages, Paladins, Priests, and Warlocks all have buffs. Buffs are bonuses that can be cast on other players. They typically last 30 minutes, but some are shorter.
Druid, Hunter, Mage, Paladin,
Priest, Rogue, Warlock, Warrior
Druid
Armor: Leather
Travel Abilities: Cat form (atainable at level 20) gets a talent in the Feral Combat tree to make you run 30% faster in that form when outdoors (talent available at level 30). Travel form is also attained at level 30 and makes you run 30% faster.
Notes: Healer and good buffer. Enjoyed by groups for those reasons. Also has attack spells, but those are not often used in groups since they often end up wanting their mana to heal a lot. The shapeshifted forms can be handy but not too extensively used because of druids often being desired for healing purposes. Overall slightly on the weak side because they're easily the most versatile class (healing, buffs, attack spells, warrior mode as a bear, rogue mode as a cat, water form for water speed, travel form for land speed). Buffs are Mark of the Wild (Bonus armor and stats, and eventually resists. This one is much loved.) and Thorns (Enemies take damage when attacking the buffed character. Lasts 10 minutes.). Can remove poisons, and at some point get a ressurection spell that can be used once every 30 minutes.
Hunter
Armor: Leather until level 40 when they get access to Mail
Travel Abilities: Aspect of the Cheetah at level 20 (increases running speed 20%, if you take damage will be dazed for 4 sec), and Aspect of the Pack at level 40 (same as "of the Cheetah" but effects the whole group).
Notes: Only class with special abilities for ranged combat (Meaning with bows, crossbows, and guns. Maybe throwing weapons as well, I'm not sure.) Only class which can tame animals to be their pet and ally in combat. Pets are good for keeping enemies distracted and letting you and your party members attack at will (and at a range). Good for finding enemies with their tracking abilities (shows enemies of the chosen type on your minimap). Although they're definately one of the most popular classes, some people don't like to group with them because hunters not played well can attract extra enemies since their pet may bring unexpected attention when wandering alongside the player.
Mage
Armor: Cloth
Travel Abilities: Blink (instantly teleport 20 yards ahead, usable every 15 seconds), Teleport (teleports you to a capital. Stormwind and Ironforge at level 20, and Darnassus at 30), and Portal (opens a portal to a capital for your party members who can click it. Stormwind and Ironforge at level 40, and Darnassus at 50).
Notes: High power ranged damage dealer. Rightfully dubbed "nuker"s. Major damage output, and not very tough as a counterpoint. Buffs are Arcane Intellect (Bonus Intellect, which translates to MP, spell criticals and faster skill increases), Amplify Magic, Dampen Magic. Can remove curses.
Paladin
Armor: Mail until level 40 when they get access to Plate
Travel Abilities: Free mount at level 40 (Warhorse), and once they add it, at level 60 too.
Notes: This class is a little overpowered, and as such is very popular. They're a combonation of two major roles that normally have to rely on each other: tanking (taking the hits, which makes that role very much in need of a healer) and healing (healing classes normally need some to take and deal the hits, since they're not tough). In addition they get auras (work like in Diablo 2) and blessings (cool 5 minute buffs). They're not the best tanks (in groups they can't hold an enemy's attention like a warrior can, and they don't have any special melee attacks at all), and they're not the best healers, but with those things in one char, with their buffs, auras, the ability to resurect players, and their invincibility abilities on top, they're pretty solid. Possibly the most common class among the regular players in the guild. Can also remove poison and disease.
Priest
Armor: Cloth
Travel Abilities: None, unless you count Levitate, which does just what it says from what I've seen. I don't really know much about it.
Notes: Most loved by any party (and guilds, wink wink). The best healing spells, and also protection spells and ressurection. Can still use attack magic, but isn't very powerful with it. They also get some telepathy things (seeing through target's eyes [players or creatures], controlling enemies) and levitate. Buffs are Power Word: Fortitude (Bonus Stamina, which translates to HP) and Power Word: Shield (30 second damage shield, good for keeping group members from needing healing). Can remove poison and disease.
Rogue
Armor: Leather
Travel Abilities: Sprint - Increases running speed greatly for 15 seconds. Usable every 5 minutes. Increase amount starts at 50% at level 10, and eventually reaches 65% at level 58. There is a talent which you can put points in to reduce the cooldown up to 1.5 minutes.
Notes: The mages of melee. They have charges and finishing moves like the Assassin class in Diablo 2. They're not tough but can do the most melee damage. Can use Evasion to dramatically increase their dodge chance for 15 seconds every 5 minutes, and thus temporarily becoming tough.
Warlock
Armor: Cloth
Travel Abilities: Free mount at level 40 (Nightmare), and once they add it, at level 60 too. They can also summon other players to them (see below).
Notes: Simply put, the evil class. Very well-suited to soloing. They summon demon minions (only one at a time) to help fight. Their attack specialties are curses and spells that do damage over time. They also can drain the soul of an enemy when they die to create a soulstone. Soulstones can be used to make Healthstones (like healing potions, but free, and from what I've seen, better), bind souls (I think that's what it's called... they do something to a player so that when they die they will automatically ressurect after a short period), and I think other things that I'm not familiar with. They can also summon other players to them if they have the help of any two other players (class is irrelevant for this purpose). So if you and at least two others invite another player to your group and they're far away, you can summon them to your location. They also get a cool 10 minute buff called Unending Breath, which lets you breath underwater.
Warrior
Armor: Mail until level 40 when they get access to Plate
Travel Abilities: None, unless you count Charge. It's an abilties which can only be used out of combat. You charge an enemy (from 8 to 20 yards away), stun him, get some rage, and begin combat.
Notes: The tanks. They're the best at taking damage, and keeping an enemy's attention, while doing decent damage as well (rogues are the best for melee damage, but not nearly as tough). Along with a healer, this is the role every solid group needs filled. The downside is that it seems Protection-based warriors are the only viable ones. When a warrior is invited to a group it's to be a tank. Fury- or Arms-based warriors cannot hold enough agro, and if they could, wouldn't be as tough. They will do more damage, and get more rage points, but simply not be as effective. Blizzard has recently given the warrior abilities in Fury and Arms new abilities to try and make up for this, but I don't know if those changes have been effective. Besides, I personally like the idea of being tougher than a dump truck while really pissing off the enemies. Such a nice change from my dangerous yet fragile rogue.