I love the starting event demo'd in the video. I think it's brilliant to push players into combat instead of picking flowers and killing some low-end monsters in some quantity. The boss fight was pretty neat - I like the dodge mechanics, I like the scaling dificulty. The skill leveling (by weapon instead of level) is neat... but I wonder if the skills are the same for all weapon types (sword, gun, hammer) or if it changes based on the name of the weapon.
When the video transitioned to the under-water fighting I wasn't very impressed. It looked as lame as other underwater combat mechanics. I like not drowning. I like being able to sink an enemy, but I feel like that's a silly skill since you can't drown them as they sink. W/e.
Oh, the dragon was beast. I loved watching the island/castle crumble beneath him. The wall of bones was pretty cool. I like how there are half a dozen different things you need to do to win (defend cannons, raise engineers, damage dragon, fight mobs, etc.)
PvP map looked weak as hell. "Destructable environments..." looked cheesy as hell. I feel like I might be disappointed with a lack of World PvP.

I think of sinking players like an equivalent to the distance gain moves on land. But under-water fighting is what it is, there isn't much you can change with it. I think you need to have seen the live pvp video of ArenaNet vs Boon, but it was on around 7am central since this is in Germany. Destructible environments seems like a cool idea and as you can see it does change how you might play a map.
Asura laugh animation at 28:12 was funny to watch and Guardian skills look neat. Especially dat knockback.
The only thing that bothered me in this video is the jackass who kept saying mkay mkay mmhm mkay mmhmm mkay mkay every 30 seconds.