Since we're less than a week away from Cata, I've started planning which characters I want to play and in which order. I couldn't help but notice that I'm very influenced by my choice of professions for said characters.
First and foremost, I ought to consider which class mechanics I'd rather want to work with for a while. However, after playing all healers and almost all tanks (save warrior which is reserved for my goblin), I've just realized two things. One: the grass will always be greener on the other side, no matter what fence I'm on. Second: Barring broken class mechanics, I can work with almost any class, and I consider them pretty balanced too.
With that in mind, I've started thinking about which professions I'd prefer to have maxed out first. There are several things to keep in mind here. The first is that (practically) every profession offers useful stuff for that character itself. So while some of them look really neat and cool, I try to disregard that information when evaluating professions. Instead, I'm looking at what each profession can provide for my other characters.
AlchemyI've decided that I will be leveling my paladin first. It sounds strange for me to say that, but I really enjoy paladins now with the new 4.0 mechanic, and since I've paid for a race change, I no longer have as much trouble with the theme. In fact, I think Sunwalker is a cool theme. I have Alchemy / Herbalism on my paladin, which is a very nice thing to level first. After all, everyone can benefit from elixirs and flasks.
In addition, alchemy also bring some other gems, namely with Transmutation. Since transmutation has a 1 day cooldown, the faster I can get it the more items I can get from it. Add to that that my paladin Koiane (formerly Tirvin) is a transmutation master, meaning I can gain more than what I started with using transmutation.
There's also the Potion of Treasure Finding, which all my alts can benfit from.
I'm not going to hide the fact that I'm really excited about the ability to transform myself into a dragon. For that particular reason, I've decided not to spend time getting a nice-looking flying mount for my pally. However, ... personal benefit only, and it's only a cosmetical upgrade unless I'm going to carry friends around.
InscriptionMy other full hybrid, Ayeba (druid), has inscription / herbalism. It is a very useful profession, but just not as useful anymore after 350. As far as I can see, there are no new glyphs. There are an almost-as-good-as-Fortitude-runescroll, a book that gives you a random buff on a 4 hour cooldown, the ability to create darkmoon decks and relics and off-hands. Only my priest will benefit from off-hand items apart from the druid herself. I've got several characters who can benefit from relics, though. However, it will grant no benefit if the other characters are decently equipped, then the items crafted will be stricly worse.
The interesting part for me is that I can level inscription with herbs from Koiane without actually leveling Ayeba. However, I will probably level my druid anyway, as it's a neat class in itself.
EnchantingMy death knight is enchanter / miner. Enchanting will be very useful for all classes at top level. Less useful while leveling, though.
Mining is also handy, because it can provide me with raw materials for...
JewelcraftingJewelcrafting is still a top profession. Gems will always be usefulf or all classes, mostly at top level. Unfortunately, I've got JC on my shaman, which I'm not planning to level anytime soon. Right now you can call me tank-a-lot, and shamans don't qualify. However, if I spend time on my death knight, I can level the jewelcrafting without actually leveling my shaman.
EngineeringMy priest is an engineer. I've had an awful lot of fun with it in the past, but unless I've missed something, it's still a very 'egoistical' profession that offers little for alts/other classes. Okay, there are a few new cool group benefits, but they require the user to be an engineer himself or herself, meaning I can't send it to alts.
I must admit I'm really tempted to play my priest just for Leap of Faith. However, no tanking here either (in fact, less so than with shamans), and other classes also get cool new stuff.
BlacksmithingI don't have a blacksmith... yet. However, give me a few weeks and I'll probably have a goblin warrior one. (If you've been reading my journals, I decided to make my goblin a blacksmith instead of an alchemist because I now have an alchemist, namely my paladin). However, I'll need months to get it to top level, if I'm going to spend any time at all with my current 80s.
Blacksmithing is a semi-useful skill. It has the potential to be useful for my plate-wielding alts, but requires that the blacksmither becomes 85 before the other plate-wielders. Since that's not going to happen, blacksmithing will only offer stuff for personal benefits. Correct me if I'm wrong here. The only item I know of is the "enchant" that grants an extra gem slot in a belt item.
Leatherworking, Tailoring, SkinningGot none of either, so not applicable. My wife has a skinner / leatherworker, but she's not playing WoW now and possibly never will again. Both leatherworkers and tailors, however, suffer from the same problem as blacksmithing and to a certain degree inscription 350-425: They grant only benefits to lower-level alts. With the exception of the leg-specific enchant.
Bottom lineAlchemy, Enchanting and Jewelcrafting are, in my opinion, the top three professions for helping your other characters. All three continue to help other classes even when they're all at 85.
The other professions can help lower-level alts, but grant very little once you're all at 85.