Although the World of Warcraft MMO has been around since 2004, it is still possible that you are a first time user trying to decide which Alliance race to choose, or maybe you have been playing Horde from the beginning, or maybe you were sticking to a single Alliance race and wondering what others are all about. Whatever the reason here are some heads-up about the post-Cataclysm Alliance races.
Human: Humans can play any class except Shaman and Druid. Their race powers are pretty much useless except for the reputation gain bonus. Reputation bonus becomes helpful in the high end gaming. Not essential but surely saves you some hours spending gaining reputation for a certain faction. Since they have a sword and mace bonus, you may consider playing a human if you plan on playing a melee class.
Dwarf: Obvious choice for a hunter. They get a bonus for recent post-Cataclysm skill Archeology, which is not such a big deal. Also has a mace bonus. Cannot become druids.
Night Elf: Cannot play shaman, warlock or paladins. Obvious choice for rogue or druids, since they can move faster while stealthed. Also they move faster in death, which sometimes helps for rogue or druid PVP.
Gnome: Cannot become paladin, hunter, shaman and druids. Incread mana pool racial power makes them obvious first choice for caster classes like warlock and mage. Gnomes also get expertise with daggers and small swords as well as escape from speed altering effects, which spell rogue. Their engineer skills also get a bonus which is also a plus for rogues. They are small and sometimes hard to target in panic situations, which is good for PVP.
Draenei: Draenei cannot become rogue, warlock or druids. They get a bonus to hit with melee or spells and healing powers. They seem to be good choice for paladins. Draenei also have a bonus for jewelcrafting skill.
Worgen: The latest addition to the Alliance races, Worgen has a sexy starting area. You need to create 2 avatars (one for human form and one for Worgen form). They cannot become paladins or shamans. Worgen get a critical bonus which is good for druids or rogues especially. Periodical move boost is usable for all kinds of characters.