no professions are NECESSARY, but it is up to you.

it's nice to have enchanting, jewelcrafting and inscription.

The crafting professions like tailoring/blacksmith/leatherworking are ok but most times you won't be wearing crafted gear (only early on in an expansion and then late from raids).

Decide what you want to do.. it is up to YOU.

Either spend time gathering mats now while you level up or expect to pay for them on the auction house. When cataclysm hits all mats will skyrocket in price because of demand. I'd just make sure you have as much done then as you want because it'll cost an arm and a leg to level at that point if you don't do your own farming.

Ultimately you need to find out what you want to do.... do you loot all the mobs now? Do you save the cloth / white items / crafting materials? Would you rather just get to 80 as quickly as possible and worry about it later? We can't answer those things for you.