In a recent video the game director states that ESO will have a moddable UI (similar to the one which WOW has).
For those of you not familiar with UI mods it means that the community can alter the UI or add more detailed information, such as the time left on an ability cooldown. This is the first piece of news on ESO that has dissappointed me greatly; here's why.
Firstly, I will admit that at first glance UI mods are a positive feature, and if you don't like them, you don't have to use them. If one desires, the UI can be as complicated as possible with every detail of combat covering your screen in numbers. Or it can be left as the developers intended. This is only true of the casual player, who does not care if they are optimal, or in other words, if you care about being good, you will use UI mods (if they are available).
This combines with the second issue, and why I was more excited before hearing this news: The UI Zenimax has spoken of was designed to be minimalistic; to add to the immersion of the game. Ideally, the systems in the game could be represented without clutter on your screen during combat. Having UI mods available, and knowing that others are using them while you are not, puts you at a gameplay disadvantage, especially in PVP. In order to not be the 'svcker', I will be forced to also adopt many UI mods so I have all of the combat information available, at all times. For anyone who has pvp'd in WOW, they will know what I'm talking about.
This also leads the game to have a meta-game, which is: Who has designed their own UI the best. Instead of just playing, knowing that everyone is on an even playing field, it adds the concern that maybe you died just because you were missing that one new fancy UI mod. This is the main reason many people hate UI mods in WOW. WOW UI mods also become broken after every major game patch, and I have dreadful memories of spending 2 hours reinstalling mods and configuring the options. I want to quest and be immersed, and react in real time to someone swinging a sword at me, not feel obligated to download a mod that tells me a sword swing is incoming, then have it break when a game patch comes out.
Overall, I'm surprised that Zenimax has looked at the current MMO landscape and said that a moddable UI was a good idea. Especially considering that they talked about the minimalistic Skyrim UI as a positive thing (i agree) for the game.
TL;DR: ESO developers have stated the UI will be community moddable; this is bad, and will essentially clutter the screens of every player who wants to be good.