Most everyone complains about rep grinds. They are long and boring for the most part. One thing that nobody seems to bring up is how great they can be for making gold.
Lets take Cenarion rep in Outland. You can get this rep from quests obviously, but I have another great way to do it.
Go to regular Underbog. Kill everything in there. Most importantly, make sure you are picking up the Sanguine Hibiscus. You can get over 30 per run and they sell on my server for 10 gold per item. Besides the Hibiscus, you will get a decent number of boss drops and tons of vendor trash for gold. You will get greens to disenchant. You will get netherweave cloth to send to your tailor and craft bags.
You can run the regular version for rep up until revered.
After that, run the Heroic versions of all three instances. My favorites are Underbog (for the reason listed above) and Steam Vaults. You will get Coilfang Armaments in Steamvaults which you can either use for Rep yourself, or put them on the Auctionhouse for another chunk of change.
Another rep grind that I recently did was the Timbermaw rep. It probably took me an hour total. I did all of the quests available, killed mobs until I was almost revered and then turned in 10 stacks of feathers to hit exalted. In this area there are several mithril nodes if you are a minor. I also managed to pick up 6 stacks of Mageweave cloth which I will turn into shirts later.
You can make very good gold and get these things out of the way at the same time. The gold per hour isnt great, but it is at least a fun way to do it!