Mining seems to be the most profitable of gathering professions. Ore has two consumers; blacksmiths and jewelcrafters. And gems are consumables, so there's a lot of demand. I imagine Herbalism works out pretty well too, but I know for sure Skinning sucks.
I've been working mining on my Hunter alt as she levels up. It got significantly easier to level mining in 2.4 because smelting now gives more skill points. You can get about 175 points solely from smelting easy-to-find ore. In most cases the bars sell for about the same prices as the cost of the ore, so it's free. That means you only have to whack 200 nodes to fully level mining. Boring if you do it at level 70, but totally easy if you do it as you level up.
Here are the Smelting recipes along with what range of mining levels they're good for levelling up. I've capped these at the point they turn green. You can still get a few more points afterwards but it gets tedious.
300-307: Fel Iron
The big flat spot is 270-300. You're probably anxious to get to the Outlands, but you need mining 300 to mine anything at all out there. I ended up having to do a couple hours of purely mining before I could head through the Dark Portal. A pair of mining gloves can help you out a little.