Some suggestions if you've somehow got a time machine or are playing on some sort of expansion locked server:

Vanilla: Farm Runecloth in plaguelands. As a tailor, Felcloth and Mooncloth can also be quite easy, particularly for alliance. Alternatively buy up a market and resell it once you've got a good pre-existing cashflow. Gathering at all in vanilla was pretty potent, and it was before there were major benefits to picking up a crafting profession that wasn't tailoring. Herb/Alch can also be very profitable in vanilla. If your buddies were good at dungeons, chain run them and soak up all the crafting mats. Wool is a bottleneck for horde tailors as they didn't have a dungeon that dropped the material nearby, and leveling areas around Wool's level for Horde areas are absent humanoids that drop them, where Alliance has Redridge and Westfall as well as Deadmines and Stockades. If you're a daring horde rogue, it's worth it to stealth in Stormwind or Westfall and run either dungeon or just farm the mats nearby. (frustratingly, many of humanoids in Redridge are tagged as friendly to Horde players, so go to Westfall.) Arcane Crystals drop from an elite mob in Un Goro and sell well on the AH. Crusader Orbs drop from mobs in Strat and Hearthglen. (A personal favorite farming location for me.)

BC: Standard fare from above, with Runecloth being even more in vogue due to rep turnins. Primal Mooncloth, Shadowweave and whatever the other one was (I forget) were also sought after cooldowns. (people would pay you for yours, and the crafted BC cloth gear was best in slot for many well into tier 5.) Khorium a big deal, and since crafting professions got more perks, more people were going crafting, meaning gathering started being in even more demand. JC becomes king, especially late in the expac if you have access to BT/Hyjal. Rep items (Scryer/Aldor) also a big deal. Getting Orbs from Heroics so you could make gear for people was also huge. Various elemental crafting mats are easily farmable and heavily farmed now that it's split into motes and not a fairly rare chance per mob. Get a flying mount early and get to Elemental Plateau. Twinking a pretty big deal, and you could make bank running twinks through lowbie 5 mans.

Wrath: Cooldown crafteds in very high demand. Titansteel a big deal. (component for Chopper and Jeeves as well as a host of other important things.) Relics of Ulduar a big deal. Fishing becomes a very big deal too as the feast is made via fish. When Glyphmas hit, prepared people could make a bundle. Reselling BoE epics is something more people got involved with. Green leveling gear also started spiking, particularly for properly stated items in slots that weren't covered by heirlooms. Low level crafting mats also a big deal as more people than ever are raiding and coming to the realization that crafting professions are mandatory for high level raiding. PVPers start selling 3s or 5s carries.