What should professions should I level?

TL;DR.  If you need the too long didn't read version, choose two gathering skills and do dailies.

There is a small amount of speculation, assuming that in the main similar themes from the WoLK expansion will continue in Cata. For 'breaking news' on cata tradeskills, follow Kaliope's site.

See this post on why to take an alt.  In my opinion, taking an alt just for new tradeskills is a bad idea.  That said, I am on my 5th alt working towards 80.  (A level 1 bank alt is always a good idea)

Before you can know what professions to level, you need to know what type of player you are.  Not only that, I am not even going to tell you what to level.

Some players like their reward now. It doesnt matter too much how much you get as long as you get it reliably and quickly.  You think that you could make more gold but can't/won't be bothered.

• If you are levelling your first toon, take two gathering skills (skinning, mining or herbalism) and gather while you do your dailies. You can now have both mining and herbalism on your minimap at the same time.
• On a primary raider you should still take at least one (preferably two) tradeskills - possibly something that traditionally has a cooldown, like Tailoring or Alchemy. Use your trade cooldown regularly - you can also sell these on trade. 
• One alt (assuming you have one) should be the same as the the first levelling toon just above.
• Add fishing for a diversion. The fishing daily, with it's vendor bait has the best return of all the dailies
• Sell your gathered stuff on the AH for the 'mid range' of what you see it listed for.
• Mail all your gathered mats to a bank alt, and post them on the day you gather.
• Take cooking (getting a leg up with training using the Pilgrims festival), do the cooking daily and sell all your cooking tokens - especially cata tokens (there is a surplus of northrend tokens at the moment).

Some players know it is better to wait a little for a bigger reward.  You want more gold/hour than instant gratification.  Large scale manufacturing is not for you, nor is the idea of taking up trade wars

• On a first levelling toon, consider either two gathering skills, or possibly taking enchanting into the mix.  If you take enchanting, you are going to disenchant lots of items (many bought of the AH), and will think about selling those enchants you do make using enchant
• On a primary raider you should take two tradeskills - possibly something that traditionally has a cooldown, like Tailoring or Alchemy. Use your trade cooldown regularly, buying and selling the mats yourself.  Keep an eye on regularly bought items - starter PVP or raiding gear and sell them
• Diversification is better than duplication.  On different alts take different skills.  I would recommend no more than one farming toon (per realm).
• All max level alts should have maxium skills
• Mats are not free - even if you farmed them. However, having an alt with gathering skills lets you control whether to spend gold or time.  On an alt take two gathering skills (skinning, mining or herbalism) and gather while you do your dailies. You can now have both skinning and herbalism on your minimap at the same time.
• Add fishing for a diversion. The fishing daily, with it's vendor bait has the best return of all the dailies.
• Sell your gathered stuff on the AH for the 'mid range' of what you see it listed for.
• Mail all your gathered mats to a bank alt, and post them on the day you gather.
• Take cooking (getting a leg up with training using the Pilgrims festival), do the cooking daily and sell all your cooking tokens - especially cata tokens (there is a surplus of northrend tokens at the moment).

Some players want it all.  You are working towards a gold cap (or at least wondering if you should). Bring it on.

• You are not on a first levelling toon. Really.  You might be an experienced player doing 'rags to riches', but you are still not on your first levelling toon.
• Tradeskills with lots of complications and daily quests and research are best.   In Wrath, Jewelcrafting, Inscription (and even cooking at the start) were all good options.  (edit) Enchanting isn't bad either.
• You know that you can make gold from any profession.
• Alts may start off with gathering skills but will end up with unique trade skills.
• You probably know most of this anyway because you spend as much time reading gold blogs as you do in the AH.

I will be writing another post on how I would make gold by skill as the above people.  This will be done Soon(tm).