Which profession to pick for WoW WoTLK?

Let's start with the gold-making part for professions. Because extra stats it's cool, but having a helpful profession is a great deal.

Of course, the best gold-making combo is Mining and Herbalism, but let's make it useful and still profitable in terms of gold-making.

In my opinion, the best combo will be Mining and Blacksmithing.
At the start of Wrath of the Lich King expansion not too many melee and caster weapons, so you can get a ton of gold with BS weapons like - Titansteel Guardian, Titansteel Bonecrusher, Titansteel Spellblade, and finally Titansteel Destroyer. These weapons are very decent and start of expansion for PVE and PVP players, so they should be in demand. And also you will get +2 additional sockets - Bracer and Gloves slots. Also, you can try to sell Eternal Belt Buckle - it should be in demand during all expansion.
Mining will provide you a stable income because WotLK it's about titanium ore, Saronite Ore, and Titansteel Bars.
Also, the good point will be to change Mining for Enchanting, but it's more expensive since you need a lot of disenchanting materials for leveling it, but it's also good. Weapon enchants and disenchanting will provide you with a good amount of gold at 80 level.

Let's speak in terms of min-maxing and obtaining bonuses from professions for yourself because gold-making things you can easily use from your alt characters.

The best combos will be Blacksmithing and Jewelcrafting, Blacksmithing and Engineering, Tailoring and Engineering.

Blacksmithing and Jewelcrafting
It's easy to play and provides you the best stable bonuses - 2socket slots and 3 jewelcrafter gems. These gems are better than gems that you can buy on AH. For example Bold Scarlet Ruby gem - 16 Strength, Bold Dragon's Eye 34 Strength. As you see it's an x2 stats bonus and with extra 2 gem slots, it's insane and does not require keeping an eye on procs. This combo is the best for Warriors, Holy Paladins, Frost Deathknights, and other classes if you do not want to depend on procs or active bonuses. And it's the best combo for PVP because it's easy to get a resilience boost since the start of the expansion. But the main problem with this combo is really expensive to level it up.

Blacksmithing and Engineering
It's a bit tricky because it's mostly for PVE and the best bonuses will get not every class, but it's still ok if you do not have a lot of gold for pumping BS+JC.
The bonuses: 2 socket slots and insane engineering enchants - Nitro Boosts (24 critical strike rating and movement speed boost), Hyperspeed Accelerators (This enchants it's why we want this profession. It provide 340 haste rating for 12 sec every 1 minute in use on your gloves item slot,), Flexweave Underlay (23 agility on Back) or Springy Arachnoweave (27 spell power on back). Also Engineering will give you a lot of bonus damage with grenades and sappers for PVE and Battlegrounds.
In short, it gives to you fewer bonuses than BS+JC in the static mode, but in burst mode, you will be a way higher. In my opinion it's good combo for Unholy Deathknights and Retribution Paladins, Rogues.

Tailoring and Engineering
All bonuses from Engineering that we listed before and some Tailoring enchants.
Useful Tailoring Enchants - Lightweave Embroidery (295 spell power on Back slot with high proc chance, but cooldown about 1min), Swordguard Embroidery (400 attack power on Back slot with high proc chance, but cooldown about 1min). Also, Tailoring will give you special enchants for Leg Slot - Master's Spellthread (50 spell power and 30 stamina), Sanctified Spellthread (50 spell power and 20 spirit).
It's a good combo for Mages and Hunters. And it is pretty cheap.

What about Leatherworking, Alchemy, Skinning, and Inscription - it's bad professions in terms of min-maxing, but it is still viable if you are a fan of these professions. Sometimes they can provide you with a decent amount of gold, but it's still not the best choice.

In the end, you can choose any profession which you like, but keep in mind our bonuses overview.
Also, keep in mind in WotLK pre-patch and the start of the expansion will be a lot of fresh Death Knight and new players that want to level their professions, so you can do a lot of gold with sell mining reagents to them. Because JC, BS, Engineering will be very popular.

Good luck and see you in the World of Warcraft: Wrath of the Lich King!