I'm afraid that the costumes and dyes will give bonuses like in Luna Online
for example a costume gives +250 hp +5int and +5 wisdom
I'll try it after the reviews of the item shop.
Well, the ones in GW1 didn't...
Seriously, Anet has, within the scope of GW1, kept the items available for cash to a minimum effect on the game balance.
-Costumes fit OVER the armor and are purely cosmetic.
-additional storage panels and
-additional character slots at worst give you a tiny advantage in trading due to being able to store a bit more stuff
-skill unlock packs are only an advantage if you want it ALL - to achieve the same effect of having ALL skills unlocked, you'd need a lot of work - but to unlock the 8 desired skills that you'll actually USE for your build takes little time, even for several builds
-pet unlock packs may save you some work, but in the end are cosmetic, since levelling up a single pet will get you the same effect
-preorder&co items are nice to look at, but generally don't have the best stats
See a pattern here? I expect Anet to offer a lot of stuff in the store, but not in a direct "power for money" way.