My UI
 

Finally (and with some help from Zork) I have finished my Qulight addon's based interface. Any suggestion is welcome :) (yep, I know I should change the damage font to match the others)

Idle:


In combat:

I'd get an error filter and (I don't mean lua errors I mean the error text) and remove everything except "immune", "evade" and "out of range".

The rest is just so much red text spam with no practical use whatsoever, especially since you have combattext in that same area :)

For example the aptly named ErrorFilter.

I can't say I like the unit frame placement, especially the target-of-target frame being right in the middle of the screen. They're just blocking too much of the important parts of the game world where they are, and the ToT frame will literally cover up your character's head when you're not on a flying mount. I'd move them down a bit, and maybe move the ToT frame out of the middle.

Also, you need to work on making the spacing between elements consistent. For example, your chat frame block is about 20px away from the left edge of the screen, but about 2px off the bottom edge. On the other side of the screen, your Recount block is flush against both the right and bottom edges. In the middle, your action bars block is about 5px away from the bottom edge of the screen. At the top, your raid (?) frames and buffs are about 10px away from any edge of the screen, and your minimap is about 20px away from the screen edges. Try to line everything up so the spacing is consistent.

Ok, I changed the Minimap, Chat and Skada positions to be at the same ditance from borders than Raid frames. Didn't move the action bars as I don't feel it needed for being in the middle (they aren't assimetric with anything). Also aligned the ToT frame with the pet one. I didn't installed any error filter addon for now.