Game Systems Designer Jon Peters has written a dev blog entry that outlines all of the changes the ArenaNet team made to the Guild Wars 2 demo for Gamescom. The list is extensive:

• Skills no longer cost energy. This means energy potions are also gone. In place of the energy-potion button is a new dodge button.
• A weapon’s skills are now learned by fighting with that weapon. Non-weapon skills will be unlocked using a collection mechanic similar to Guild Wars.
• The Gamescom demo is the first time you'll see the story from the charr perspective, as well as the charr starter area.
• Both asura and sylvari have the chance to explore the high-level area of Sparkfly Fen.
• All of the big, giant bosses have improved; from better cameras to view them to new things being done with their skills.
• You can now try out our character customization (seen above). Each race has its own unique set of options, including charr horns, norn tattoos, and asura ears. There is a large variety of faces and hairstyles, and you can even customize facial features, including five sliders just for your human character’s nose!
• Almost every weapon for every profession has changed in some way or another.
• Structured PvP is playable in the demo. ArenaNet is introducing their first map, the Battle of Kyhlo, a conquest map with siege weapons and destructible environments.