Unless you quests heavily and/or do a fair amount of gathering as you level, you might find yourself lacking gold at lower levels. Once you hit 80, you probably want to get Artisan Flying and start thinking about Master Riding. But unless you have gold lying around from alts, you’ll probably be far off from the 4-5k gold for making the purchase. So how can you build up some gold early on?
One of the best ways I’ve discovered is to do some Outland instances. Places like the Blood Furnace, Hellfire Ramparts, Mana Tombs and even Shadow Labyrinth can be decent places for making around 100g/run. As an 80, you should be able to handle most of these (although Shadow Labyrinth might pose a slight risk depending on your gear). I tend to do Hellfire Ramparts and Mana Tombs as they aren’t too difficult. Hellfire Ramparts are great because it still drops Runecloth, which can fetch a decent price on the Auction House. Most of the money is made through the drops in these instances and selling them directly. In that sense, the gold is fairly reliable.
Once you hit 85, you can run Halls of Lightning which nets around 100g/run. But the great thing is the decent drop rate of Glyph of Mastery. Also, these places drop Frostweave, which still can fetch a decent price on the Auction House. Lastly, you could farm the area in Icecrown where the groups of mobs drop Frostweave.
Overall, I tend to prefer questing to make gold as I level up. If you instance grind while leveling, you can still make gold but not nearly as much as questing at least in my experience. Both the quest rewards and drops from mobs tend to provide better gold. Yet if you want some quick gold at this point, running those instances mentioned here will go fairly quickly.