Blizzard has designed World of Warcraft so that each class is (to an estimated) balanced and not overpowered. Tweaks and fixes are constantly made to classes' abilities and talents to ensure that World of Warcraft is a fair game. So which race should you choose?
When compared to other MMORPG's, World of Warcraft offers very few classes to choose from. Each have unique habits and abilities that define their playability, enjoyment and importance in group combat. Sections would be completely unbalanced if either were to lose a class, so each one is highly valued in the land of Azeroth. They include (available on both factions):
The warrior is, without doubt, the most powerful class in the melee area. In raids, they are usually used as a "tank" – the role in which one or two players build up the most "aggro" and take the majority of the battering from raid bosses. It's definitely a fun class to play, particularly in PvP combat (Player vs. Player).
The Paladin is viewed as a major annoyance in the game, mainly due to it's unique protection and healing abilities. Paladins are able to "bubble" and effectively become immune to combat for up to 12 seconds, during which they will either heal back to full health or avoid combat completely by hearting back to their home.
At this time of writing, the warlock is considered by the majority of World of Warcraft's community as overpowered, thanks to it's fear and damage-over-time (DoT) abilities. Calls have been made to "nerf" (the process in which a classes' talents and / or spells are altered) the warlock, and as Blizzard usually succumbs to the community's requests, it might not be wise to go with the warlock until this nerf has been made.
The Hunter is, in my opinion, one of the more balanced and fun classes to play. You're able to tame a pet at level 10 and utilize it as your very own tank, all while you stand back and make the most of your archery skills on the target. They are not highly requested in raids, however they are extremely powerful in PvP combat.
The rogue is also a melee based class, but relationships on special attacks rather than brute strength. Rogues make use of "stun locks" – unique abilities that lock their target and freeze them while they slice and dice away. If you're looking for a fun, action packed class, go with the rogue.
A huge amount of responsibility is put on practices as they are primarily a healing class. They are usually in high attendance in raids. The downside to sacrifices is that you'll rarely ever be needed for damage, and so your role during your playing career will be varying between the same three healing spells.
Mages are a high damage class that is on magic spells to defeat their enemy. Compared to other classes, mages have relatively low HP (hit points) and are highly vulnerable thanks to their easily penetrable cloth armor.
The druid is a hybrid, all-rounder class with the unique ability to transform between animal and man. In animal form, they are able to deal a high amount of damage but are vulnerable to high damage and special attacks.
The shaman, previously only available with the Horde faction, is similar to the druid but is able to deal much more damage through nature-inspired spells and attacks.
If you're a first time World of Warcraft player, it might be best to try out each and every class for a day or two before deciding on one and continuing on with it. You will invest a lot of time into the character, so choose wisely!