Gear required for WOW power leveling is different from gear required for raiding, heroic instance or hardcore PvP. Don't get too pressure out about it, because you don't have to be a gear expert if you want to participate in endgame instances. Each class has different stat requirements, but they also have different dependencies on their gear quality. For example, I have found it much more important to keep the gear current while leveling my rogue as opposed to do Wow powerleveling my mage.
Here is a gear guide in general for collecting the best gear while casually wow leveling your character(s). You should also try to keep up with what stats are best for your specific class and spec.
Basically greens are fine for WOW leveling and try to pick up blues from quests and instances when possible, otherwise they are not worth the extra cost. Tips: don't sweat the purples, because they are not worthy. If you get a world drop, it's better to make some wow gold at the auction house than to wear the item and replace it a few levels later. The best thing you can do with it is to hope for a purple-crazy sucker on the AH.
Check out the rewards before you deciding what quests to do. Some quests offer decent WOW leveling greens and blues. If you are Horde between level 10 and 20, the Ghostlands quests and the accompanying Tranquillien reputation will outfit you nicely. If you are 58 to 70, the Outlands quest rewards and drops are huge upgrades over the old world gear and you probably won't have to spend any WOW money on gear at the Auction House during this time.
Instances are great for gear, World of warcraft gold and experience. But the instances you encounter while do world of warcraft leveling can be frustrating time sucks with the wrong group. And we just don't have the time to waste. If you have competent guildies/friends with characters near your level and similar schedules, you have hit the jackpot. Otherwise, your best bet if you want the instance goodies is to get someone to run you through them. Personally, I think it's dullsville to be run through instances, but The Spousal Unit and I have had some fun taking turns running each other's alts through dungeons for gear.
Professions are a great way to outfit your character while wow leveling. If you keep your professions at or above your level, you will always have goodies to wear or improve what you are wearing. If you are a mage, see the Arcane Brilliance series on professions. Also, pay attention to the professions your guildies are grinding. They are usually more than happy to provide you with free crafted gear rather than vending all of their products.
The Auction House
Every 10 levels or so, take inventory of your slots and see what items are pushing obsolescence. Some blues can be worn for 20 levels, but most greens should be replaced after no more than 10 levels. Then go to the Auction house with a slot list and shop for greens that are improvements and are at or a level above you. Again, don't waste WOW money on Bind on Equip blues and purples. Your cash is better spent on crafting and mounts.
Hop over to the WoW Armory and find your character, if you want to spend some time and effort micromanaging your gear for WOW powerleveling. Mousing over each equipment slot will show you the full stats of the item you are currently wearing along with where you got it. For example, I'm wearing the Robes of Arugal on my priest, which I got from Archmage Arugal in Shadowfang Keep and has a medium drop rate. If you mouseover the arrow on the outside of the slot, you get a button to push that will check for upgrades. If you don't like the level range of what you are seeing, you can click on Show Item Filters and adjust the search to suit your needs. From here you can make a shopping list with which you can plan your questing and/or shopping.