What to do with skill points once you've unlocked all skills?
If you go to Lion's Arch and talk to the NPC Miyani next to the Mystic Forge, she has various items for sale that you can purchase with Skill Points. The most expensive is a Bloodstone Shard, which costs 200 skill points and is used to create Legendary weapons. Currently, that is the only other place to spend skill points aside from on your character.
In addition, you continue to earn skill points after you hit the level cap of 80, gaining another when you would have normally leveled up again, similar to Guild Wars 1.