I wrote this on some general forum thread, but thought it would be better to copy over here. Also I revised it a bit after some thought.
Problems I have with the UI:
A) I preferred having the spells on the left side of the screen because that mirrors your hand placement making it more intuitive especially when learning the game. Having all the spells on one bar is not good because then its harder to single out at a glance. The addition of the passive button is nice however I don't see why you need to dedicate UI space for something that you cannot activate. After playing a hero the first time you will know what their passive is so it becomes even more superfluous.
B) I really hate the dedicated stat box, its an eyesore, for the amount of time you spend looking at stats it should not take up so much screen space.
C) You don't really need the stuff on the upper half of the screen especially now that there are large portions of unused space on the bottom. Another thing is that you have THREE personal health/mana bars on the screen, why do you need that many? Most of the time the only bar I look at is the one over my head.
My layout suggestions:
1) Make the ability buttons back into what they were in the old UI but just keep them below the "horizon" so they do not intrude into the main play area. Add a level indicator and put your experience bar underneath everything. Essentially this would be the "spell and level up area".
2) Combine the bottom health bar with your character portrait to the left of it, put all your teammates boxes under that box and size everything specifically so together they make one large rectangle. To the right of that make a mirror box that shows your enemy target information.
2a) When you don't have a target then the mirror box shows your item bar and personal statistics, two things that in my opinion you do not need to see 100% of the time so they would be much better this way.
3) The K/D/A window should be a little bigger, and it should be cut into two parts with the match time separated making it easier to see. I also think the gold counter should be up here was well making it stand out more and to save room on the bottom of the screen.
4) I think it would be a good addition to have portraits up for the whole enemy team somewhere on the screen with their level, rez timer, and death indicator. The information is available in the score window so I see no reason to not include it as part of the main HUD.
In closing I have included some awesome MS paint pictures, one with target one without.