Of course I don't think they worked on it for a year, that's my complaint that they didn't work on it for the past year, to be able to move large chunks of the UI was a decent start back then but far from a viable solution to the horrible UI issue.
I know why people left, I can tell you that the horrible UI surely didn't help to keep any players though, even if it wasn't their primary reason for leaving.
If you look at my screens you would see that it's not a solution...
Why does content have to dictate having a functioning UI?
There is no harm in learning what visual effect means what either, same as there is no harm in playing the game only ever using two buttons. Doesn't mean it's a good way of doing things nor that it's enjoyable to everyone. And raiders do have issues with lack of clear representation of information within their UI, being a raider I know this for a fact, same as they have issues with bugs and lag, one does not exclude the other.
Over a year after release of a AAA MMO you would expect both lag and bugs to be under control to an acceptable level as well as having a UI that offers comfortable experience playing the game at any level, especially taking in to consideration how horrible the UI was at launch.