There are new things to gather, but the gathering professions haven't changed. Demand for green drops from mobs will probably be strong for awhile since much of it is better than the epic stuff people had at level 70. Certain crafted items (especially glyphs) will sell very well and the materials to make the popular enchants and crafted items will sell very well. The only real difference here will be in exactly which crafts and enchants are popular.

Skinning - Skin everything that you can, as always. Sell the scraps, too. There will be hot crafting recipes and skins needed to supply the crafters, keep that in mind when scanning the Auction House.

Mining - Cobalt and Titanium await you. Engineering, Blacksmithing, and Jewelcrafting will create a demand for these ores and that demand should stay strong for a long time.

Herbalism - Now that Wrath of the Lich King is here people will get back to raiding which will require the usual collection of potions, created from the various herbs that you will be picking. The new profession, Inscription, gets its inks from herbs which has resulted in herb prices going way up.

Much of the playerbase has moved into Northrend and will start farming soon, which will cause the prices of the new herbs to drop. Outland herbs, which have been cheap for some time now, might go up as the gatherers head to Northrend.

Fishing and cooking - Some fish will likely be needed for Alchemy, as well as for cooking, and some might turn out to do interesting things (such as Savory Deviate Delight.) Of course, certain foods are in demand for the buffs they provide and those foods should sell well.

Looting - Keep a close eye on any white or better items you loot in your day to day monster mashing. Some of these items will be valuable. Send all of them to your banker for resale.

Limited items - I have no doubt that there will be a number of new "limited items" appearing on various vendors. Keep an eye on these as several might sell for a very nice profit on the Auction House. As you find these vendors make a note of their locations and build your own vendor list.

Speaking of the Auction House... This really is the area where people can make some serious gold. If you have a good understanding of the market on your server then there are two basic Auction House ideas to work with.

1) "buy low and sell high" is a tried and true technique. Take herbs for example. If an herb (or ore, or...) normally goes for 30 gold a stack and someone is dumping stacks at a very low price then yoiu have to opportunity to buy them up and resell them. If prices are starting to look depressed then just hold those items for a few days and keep a close eye on the AH. When the market allows (that is, when prices go back up) you post them at a higher price.

2) Find holes in the market and fill them. If a certain glyph is rare and selling for a high price then see if you can make a few of that glyph and sell them for that high price. Same with herbs. If the market needs a few more of a certain herb, because prices are so high, go out and farm a few.

Lastly, consider using a low level alt to do all of your buying and selling. It's less distracting than if you use your main character, you'll be able to focus on making gold and not spending it on better stuff. Just make a level 1 character to be your "Banker" and run to the nearest big city and park in the Auction House.

Next have your main character mail this character some starting money (for auction fees.) Then as your other characters collect sellable loot and limited items they mail all of those items to the banker for resale.