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.
Best Answer
You only get to see the dungeons within your level range. If you exceed the max level of that dungeon they will disappear from the dungeon browser. Heroic dungeons have a stricter level requirement than normal dungeons (max and min level), so they disappear sooner.
In your case you have reached the range of WOTLK and only get to see the heroics from there. And you're too high for BC and too low for cata heroics, so they're not there. You can still access lower heroic dungeons by going there directly, but not queue for them on your own
*.Some examples:
Level 90
Level 110
*- You CAN join the other dungeons.Go there directly. Just walk in and you can clear them (solo or with a pre made group).
Group up with someone within qualified level range (e.g. level 70 for BC heroics) and start a party sync. Let them queue up for a dungeon and you can join them. The dungeon will be scaled to match your level (wihtout affecting the other people) and you won't be able to use skills, talents, etc higher than allowed by the dungeon, so you should feel perfectly like you're in a "real" dungeon. Due to the low amount of people doing this you have to expect longer waiting times.
Time walking weekly events work as well. They basically eliminate the max level requirement. If a certain event is up you can queue for it and it will be exactly like the heroic version of those dungeons, except that you have all your current skills and spells available. Higher gear will be scaled down to match the level of the dungeon, so they can't be outgeared that easily.