Having never really seen or played WoW, does the interface really take up 40% of your screen like that? If that is what people are expecting out of a UI overhaul in XIV, SE can consider my subscription canceled, cause that is a cluttered and disgusting eyesore.
WoW's interface is extremely efficient and effective, having said that most WoW players also agree that the default GUI is in fact horrible ( but much better then XI's..). However due to blizzards extreme cooperation with addon developers,a GUI can be customized to virtually anything you could think of (indeed there are GUI's out there that look nothing like the default, some are quite pretty as the author clearly put a lot of effort into their art design).
For instance, my hunter had around 15 important abilities routinely used, probably 10 more situational abilities, and a few more odds 'n ends that rarely got used. Despite this, I had no buttons/action bars visible on my UI taking up space as I used an addon to key-bind every last one of them (thus there was no need to click soemthing onscreen, and therefore no clutter). The only things on my UI were Player Health/Mana bar, party member Health Health/Mana bar (only in group and I had them fairly small), inventory button (which for me was one unified box instead of seperate bags), menu, mini map, chat log. With a 1920x1080 resolution there was virtually nothing on my screen. There were actually some UI's that went to an even greater extreme of minimalism and had nothing on the play-field at all.
An early example of this UI I mentioned (before I finished customizing it): http://i529.photobucket.com/albums/dd332/Quxudaic/WoWScrnShot_080710_180724.jpg
WoW was far from perfect, there's good reasons I decided to quit. However, if one can be unbiased and realize a game is just a game without getting bogged down in fanboi-ism; then one can recognize that there are also reasons why WoW has 12 million subscribers, they did in fact get many things right. The intuitiveness of the G/UI (even the basic one) is among those things. It was crisp, clean, efficient, and when you started the game you got helpful pop up messages explaining exactly what each thing does.
One of the complaints about FFXI was that its UI was cumbersome, but at the time I didn't really mind it. FFXI was my first MMO. I quit FFXI after 4 years and moved to WoW with a very close friend, I loved it and the UI but It got tiresome and there are many things that I did not like about WoW (it's community chief among them..gah..worse the XBL). Now I have moved back Home as it were to final fantasy, sadly the UI in XIV as it stands now is worse then XI's. But hopefully it will be sorted out as the game grows.
A UI, in my opinion, should be crisp, minimal, intuitive and fast. It should tell you the info you need at a glance and allow you to perform the actions you need to perform quickly and effectively. For instance, clicking the ! symbol to open the menu to click another button to mine/activate an aetheryite/log/etc is cumbersome. There's no need for that extra step. If your near a mining node, a button should pop up that immediate starts you mining, if your near two intractable objects, you should have two different buttons on screen. Frankly I liked the way WoW handled gathering, mines and herbs had actual graphics in game that you clicked to start the action. Still the bright light method in this game does aid you in finding them so it works equally as well.
In my opinion(and this may seem inflammatory on first glance but if you read it rationally I assure you it is not so much so)the perfect MMO would be a synthesis of many of WoW's efficient UI features (which have been tweaked, tweaked, and re-tweaked over the course of 6 years or so) with FFXIV's game play. To be honest, I had really been hoping that FFXIV would also adopt WoW's notion of instanced dungeons - essentially areas your party can go into to progress through kill some bosses and what not with a small back story associated with it. It provides a nice break from the tedium of random mob grinding and also gives you a sense of purpose for being there.
Anyway those are just some of my thoughts. I am neither a FF nor WoW fanboy, I have played both games and had great times in both, to me it just seems logical to take what clearly works and roll with it or at least use it as a base for building your own version of it.