Oddly enough, let's look at this from a different perspective. Deathwing, as you see him in Cata, is actually a redesign. Go back to prior versions, even prior to WoW, and you'll find Deathwing looks nothing like the Cata incarnation. The Cata incarnation though? Well, let's be honest, Guild Wars 2 has been in development for a very, very long time now. Which one came first? Well, the Shatterer's concept art had been floating around a long time before its first in game reveal.. but I wouldn't care to say which came first. Let's say it came first or even second though.
What does that mean? Absolutely nothing.
Whether we're talking about WoW or Guild Wars, the dragon made from a mish mash of solids and glowy non-solids is in fact.. nothing new. It wasn't new when Arena.net did it. It wasn't new when Blizzard did it. Neither of them copies one another, and any similarities are pure happenstance because they both just happen to be Dragon creatures made up of a mish-mash of solids and glowy non-solids.
I'd actually be able to conjure the energy to care if the fights were identical, but we've seen the fight with the Shatterer in Demos, and the two are nothing alike where it counts.