Traditionally gathering (mining/herbalism) has been a good way to get extra gold while levelling, in Cataclysm you also earn xp. As a result this has encouraged more people to level gathering and this means lower prices in the auction house. Gathering is still a good source of extra income though. If you want to make the best use of time spent gathering there are a number of factors to consider.
Night Elf – Shadowmeld (useful for ending combat and especially useful for druid gatherers)
Worgen – Darkflight (40% bonus to run speed for 10 seconds can help you get out of combat or get to a node quickly)
Tauren – Cultivation (+15 bonus to herbalism and fast gathering speed)
Goblin – Rocket Jump (Jump forward a short distance), Pack Hobgoblin (access to bank while out of cities)
There is a bit of a debate as to which class is the most efficient. While any class can make profit from gathering, it seems to come down to one of two classes as being the most efficient, Druid or Paladin. This is simply a question of which class can cover the most ground the quickest.
Retribution paladins can get a talent that gives them a 15% bonus to run or mounted speed, and all paladins get Crusader Aura at level 62 which gives a 20% bonus to mounted speed (this doesn’t stack with other movement bonuses.)
However the druid brings movement and other advantages, namely stealth and fast movement underwater. Feral druids can get speed by putting their first two talent points in Feral Swiftness which gives a 30% bonuses to run speed while in cat form (even if stealthed).
Both of these will at times give an advantage that the paladin has no answer to.
Mount up - gives a 10% bonus to mounted speed and is available at level 3,
Bountiful Bags - gives a 15% bonus to all you gather but can only be found in guilds level 23 and above.