While gw1 was great for the ability to move and scale almost every aspect of the UI, it was missing a very important feature - third party addons. I'm sure everyone remembers the discovery of Texmod and how explosively it took off, with .tpf files popping up left and right.
Wouldn't it be great if support for such things was built right into the game? Here's three examples of UI's I've thrown together with the use of addons:
Aside from changing many things aesthetically, there are also many addons available for people who like information overload, like myself.
(Admittedly I've never even used these two parts of Recount, but they're neat nonetheless):
Quite an improvement from the Master of Damage, and the Damage Log on gw1.
From what I"ve seen, the UI of gw2 looks great as it is, but that's no reason not to include an API for people to create niche UI elements which may not be included in the default. With all the great information being released lately like the mobile apps and not-sucky pve, it would be a shame to overlook such an important part of an MMO.