While there are no level restrictions to entering Wintergrasp (though if you can't fly you'll either have to take the portal or be summoned), players must be Level 80 and in a raid group in order to enter the Vault of Archavon raid instance. It's a very short raid with exactly four trash mobs who have special abilities, but are essentially tank and spank and prepare the raid for how the boss will be like. Archavon is a DPS race.
Raids will need two tanks, the necessary healers, and a whole lot of good DPS. The off-tank is needed for when Archavon picks up the main tank and uses him to mash other members of the raid for eight seconds. It doesn't even have to be a real tank, just someone tough enough to take some heavy hits. Archavon will also leap at random raid members, dealing some damage and creating an AoE cloud that deals about 2k per tick. He just has to be moved out of this cloud to allow melee DPS to do their thing. Lastly, he shoots out rock shards at random raid members, which deals damage to anyone within 5 yards of the target. If you're the target, simply stay put. If you're not, just move away from the rocks.
The only real difficulty with Archavon is his 5-minute enrage timer. Groups must pump up enough DPS to kill him before he wipes the raid. It's an extremely straightforward fight and is the easiest raid boss in Wrath of the Lich King. He drops two pieces of either Tier 7 (10-man) or Hateful Gladiator chest, legs, or gloves in the normal version, and four pieces of either Tier 7 (25-man) or Deadly Gladiator chest, legs, or gloves in heroic. The Gladiator pieces do not require ratings and can be worn when picked up. He also drops 2 Emblems of Heroism or 2 Emblems of Valor in normal and heroic versions, respectively.
There is absolutely no reason not to do Archavon every week, as he promises the best reward-to-effort ratio than any other boss. It usually takes longer to organize a group, usually a PUG, than it is to actually finish the instance. Aside from itemset drops, Archavon also reportedly drops the extremely rare Grand Black War Mammoth, which is also an Achievement. So even if you have no interest in PvP or Wintergrasp, Archavon should always be a weekly stop for the promise of free and easy upgrades.