I don't play a DK or particularly care for the default Blizz UI look, so I don't have much feedback, other than:
(1) Omen icon on the minimap is overlapping the zone name. Move it.
(2) The rune bars seem disproportionately large and bright compared to everything else. Try making them a little smaller, so their size is closer to the health bars, so they don't stand out so much. Also, unless you're colorblind, there's no need for the text on the bars.
(3) It seems inefficient to have two bars that both display your runic power (one on the player unit frame, one with the rune bars). Since everything else important in combat is near the bottom center, I'd move your player frame down to that area too, and get rid of the extra runic power bar from your rune addon.
(4) Move the pet bar down to line up with the stance bar. There's no need for that huge gap.
(5) I see a lot of action buttons with no keybindings displayed. I hope you don't click those! I'd suggest doing one of these things to avoid needing to move the mouse to the bottom of the screen all the time and click on action buttons:
(5a) Get OPie(Curse link because the author is too lazy to keep their WoWI page up to date) and put those actions on a ring -- then you can just middle-click wherever the cursor happens to be, move the cursor in the direction of the desired action and release the middle-click to cast it. OPie is also great for having easy access to out-of-combat things like your hearthstone, professions, etc. without spending action buttons or keybindings on them or opening your bags to click on items.
(5b) Set up your first action bar to swap to the second bar when pressing Alt. Then you can press Alt-1 to cast ... whatever that ability is with the skull icon right above the button assigned to 1, etc.
(5c) If you have trouble remembering keybinds without an always-visible reminder, it would even be better to bind the buttons on the second bar to Alt-1 through Alt-= to correspond with the bindings on the first bar. It won't free up any screen space like options (5a) or (5b) but at least you won't be clicking.
__________________ Retired author of too many addons.
Message me if you're interested in taking over one of my addons. Donít message me about addon bugs or programming questions.