Having the non macro-able gathering professions (i.e. Herbing and Mining) on your "main" will make life easier. I have them both on my boomkin and have to periodically check her minimap for nodes.

If you want to cover all the professions, then I would pair the professions based on the materials used so you're not splitting the same resource between characters. Blacksmithing, Engineering, and Jewelcrafting all primarily use bars, though Jewelcrafting uses more gems/ore than smelted bars. Alchemy and now Inscription will use herbs. Having the skinner with Leatherworking probably works best and with the skinning macro it doesn't matter if that character is a slave. Enchanting and Tailoring always work well together. Unfortunately there will be 11 professions (gathering and crafting) once Inscription is implemented. To learn Inscription I decided to forgo skinning in order to have all of the crafting professions.

Otherwise, if you are just looking for the ultimate benefit; Enchanting (for ring enchants) and Alchemy (for trinkets) currently have the greatest non-shareable buffs. Gear from Leatherworking and Tailoring provide a nice damage boost for new 70s but are low on primary stats (especially STA). Tailoring is useful for bags and as a supply for enchanting mats, and it's something to do with all the cloth you'll get. Leatherworking has drums which can provide a nice dps boost. Blacksmithing and Engineer are 99%/90% useless, though on a tank it is more like 95%/60%. Having a Jewelcrafter to cut your own gems is nice but not really necessary and the BoP gems and trinkets are not all that amazing. WotLK will change lots things of course and it seems all of the professions (gathering included) will grant unique, and maybe even better, bonuses.

For your group I don't want to recommend anything specific, but having a druid as gatherer is amazing if you plan to do any solo gathering. Swift flight form is godly! Also, in 3.0 I believe (but cannot confirm from work) that tailoring epic pieces are changing to generic spellpower lower than the current frost/shadow you may have your eyes on.