If you are in a position where you are only planning to play a single character with TBC professions your profession choice is very impactful, so let’s discuss what the best options are!
Variance
There is very large differences between TBC professions in terms of how much volume you can move. Generally speaking professions with high volume items have higher potential for making gold. This means primarily alchemy with potions and flasks and jewelcrafting with gems in TBC. For the other professions the main crafted output is gear, which can be good, but is limited overall in TBC, with one key exception in tailoring.
Why is tailoring different?
The main reason tailoring is different is the existence of the cooldown cloth recipes. This means you get steady profits every day from being on top of your daily cooldowns. The cooldown cloth is all needed to craft the BoP epics that are very strong in the first phase. This is a very significant bonus compared to the other armour professions, and does mean tailoring is up there.
Alchemy vs. jewelcrafting
Alchemy and jewelcrafting are pretty much the only two true volume professions in TBC with sockets being very prevalent and most players wanting flasks and potions for their raids. Generally speaking alchemy is less gold per item, but more sales as well as larger price swings on reset days. This means alchemy edges out jewelcrafting for raw potential, but it does require more time crafting and more time camping the AH to get that potential. You can go both if you have a lot of time, but both consumable crafting and prospecting is very time intensive. So you really do need to have a lot of game time to get value from that combo.
Jewelcrafting + tailoring
The lazy pick is jewelcrafting + tailoring. This gets you some lazy gold from tailoring and some relatively lazy volume crafting with jewelcrafting. Now to get into jewelcrafting you need a lot of gold. Most of the gem recipes are world drops and you want to buy at least one for every colour as fast as possible. Back at TBC launch this cost me several thousand gold, but I made it back very quickly, particularly when raids opened up and gem prices exploded.
Get your 1-300 materials
If you want to go jewelcrafting you need to stack up your materials for leveling 1-300 now. Be ready so you can get going ASAP with the new recipes as you hit max level. One final note is that due to so many of the JC recipes being world drops you can very easily have a jewelcrafter at level 60 that never enters outland outside of getting to Shattrath. So if you actually have 1 extra character then consider putting JC on your alt as the majority of the recipes are not reliant on being level 70.