This guide to getting gold is only for high level players. If you are not high level, it is recommended you focus on leveling and pick up the gathering professions (skinning, mining, herbalism) to support the cost of the mounts.
When you hit 80, it's a whole new world where getting gear is the main goal instead of getting experience. You cannot buy the most of the best gear in game with gold. However, you can buy quite a few decent epics with gold and that can help you get a jump start on doing instancing or pvp. Also high level gems, enchants and repairs can end up costing a lot. Most of all, epic flying (also referred to as Epic Cold Weather flying) costs a fortune, approximately 5000. So you are going to need lots of gold.
When you finally ding 80, there are likely to be a ton of quests left in both Icecrown and The Storm Peaks that have yet to be completed. You can earn a very good amount of gold completing most of those quests since the experience normally awarded is replaced by a higher gold amount in compensation. It is more than possible to earn anywhere from 500-2000 by finishing those 80 quests. However, like most quests, they are not repeatable so you can only do that once.
One main way most 80s earn their gold is by doing daily quests. They are quests you can repeatedly do everyday which reward a decent amount of gold and occasionally reputation with a certain faction. These can be found in more detail at the Northrend Dailies Guide. It is quite possible to earn upwards of 130 within an hour by completing around 9 or 10 of them. This can take longer when there are many players in the area competing for quest objectives so it's usually best to complete them either during low traffic hours or right after a large group passes through the area. If you are a non-dps spec (healers or tanks) you may have a harder time.
Farming primals is another sure fire way to earn a decent amount of gold. Since on most servers primal fires and primal airs are the most valuable elemental primals, the elemental plateau in Nagrand is probably the best place for primal farming for gold. It is located in the upper right area of Nagrand. Primal fires and primal airs sell for around 20-30 gold on most servers, and you can get around 5 per hour with decent dps. Unfortunately sometimes there are many other players in the area which will slow down the farm rate, so again this is done best during low traffic hours. You must have a flying mount in order to access the plateau; and again, non-dps spec will have a harder time.
Most of the time, the Plateau is occupied by three or more players all after the same mobs, so if you are looking to farm primal fire, check out the fire elementals at the Throne of Kil'Jadaen on Hellfire Peninsula. These are also part of a daily quest, but the drop rate and respawn rates are both decent, so its not a bad place to farm.
For those of you who like the auction house and like having some gold to play around with, please refer to the Auction House page.
As an enchanter, you can run low level instances for a decent amount. Although any 70 class can do it, this is best done by protection paladins and frost mages. Small radiant shards are what you are looking for. They can be obtained by disenchanting the blues from bosses in Razorfen Downs or Scarlet Monastery Cathedral. 4 Small radiants shards are required for the fiery enchant, which is in very high demand. The shards can usually go for 5-10g a piece. With a SM Cath run you can get 2 shards with an RFD run you can get 4-6 shards. You can also invite a low level player that is willing to pay you for the run's experience without looting the bosses for extra gold.
For those of you who are geared enough to do Heroics dungeons, farming Badges of Justice and Emblems of Valor can be a fast way to make some gold. The badges can be traded in for some relatively nice items. These include things like previous Arena gear, Heirlooms, epic gems and weapons. Some of the best places to farm badges pre-80 include Heroic Slave Pens (3 badges), Heroic UnderBog (3 badges), and Heroic Mechanar (4 badges). Level 8-0 Heroics all grant Emblems and the new Argent Crusade Dailies also give Champion's Seals that can be traded in for gear as well. Most heroics, if sufficiently geared, can be finished in about an hour without wipes. If you have an enchanter in group you might also be able to get shards and void crystals. Winning a roll on one of these can net you upwards of 50 to 250 in a run depending on your luck. Bear in mind that many heroics can only be done once a day.