If you're not min-maxing at the level cap, your best bet will generally be at least one, if not two, of the three Gathering professions, Skinning, Mining, and Herbalism. If you do decide to take a crafting profession, choose the gathering profession that meshes most closely with it (i.e., if you take Leatherworking, grab Skinning, Engineering, grab Mining, or Alchemy or Inscription, take Herbalism.)
If you want to make money using a crafting profession, the best choice before the cap will be Inscription, as the Glyphs you can produce as early as level 5 will be used even by level 80 characters, providing you with a constant market.
If you want to produce things you yourself will use on a regular basis, Engineering provides fun toys, and trinkets at much lower levels than they'll be availiable as drops. Leatherworking allows you to make your own armor, but drops and quest rewards will, for the most part, be better than what you can craft - at least until the Cataclysm expansion releases. Alchemy will allow you to make a variety of useful consumables, but in general, you will have to make many more than you'll be able to use as you level, and there won't be much of a market for what you craft.
Enchanting is a viable option as well, and you'll be able to enchant your own gear as you level, which is handy.
Realistically, until the +Stat bonuses at 400+ skill come into play, there really isn't a 'best' or 'most useful' profession. There are a few that will be much less useful to you as a hunter, such as Blacksmithing and Tailoring, but for the most part, you should be fine picking out whatever you find fun.
The only other major note worth making is that each of the three gathering professions provides a free 'passive' boost as well. Mining will give you some extra Stamina for free, Skinning will give your character increased critical strike chance, and Herbalism will teach you Lifeblood, a free Heal-Over-Time spell you can cast on yourself on a short cooldown. The crit from Skinning especially should be very handy for a low level hunter, though the other two perks are nothing to sneeze at either.
For the most part, Northrend and Outland are unchanged. What changes do exist, primarily are there to push players back to the old world at level 80 - i.e. the portals to the various faction capitals have been removed from Dalaran (except for Stormwind/Orgrimmar), replaced by a full slate of class trainers and an Auctioneer.
In general, WoW tends to play somewhat fast and loose with it's timeline and always has - the only real rule of thumb is that, for the most part, higher level content is chronologically later - but the Cataclysm breaks this rule somewhat as the 1-60 world now takes place some 5 years after the events of Wrath of the Lich King. Essentially, once you hit level 60, you go back in time for a little while to check out The Story So Far before rejoining the rest of the world at 80.
The most dramatic 'lore' change to Northrend is that the loading screen no longer depicts Arthas, but rather his replacement.
Best Answer
The answer here is that it depends on the profession. Leatherworking specialties, for instance, are 100% completely useless. Engineering specializations, by contrast, have a significant impact on the cost of levelling from 425 to 435, provide access to some of the coolest and most sought after vanity items in the game, and in Cataclysm, determine which of 2 new BoE pets you can make and often sell for absurd profits. Alchemy specializations meanwhile, result in an average of 20% more yield from all specialized recipes - hugely relevant, and really the only way to turn Alchemy into a substantial moneymaker.
As for Blacksmithing specifically? There are no new patterns linked to specialization in Cataclysm. Unless you're interested in some of the old vanilla or BC era BS items for vanity purposes, you can safely ignore specialization. I seem to recall a few mid 70's blue items linked to specs as well, introduced in Wrath, but they are not efficient skillups.
Tl;DR: They're irrelevant for all except Alchemy and Engineering, where they provide significant moneymaking opportunities.