Let me just be fast here and say:
There is really no tips to give to YOU for YOUR UI.
As long as it works for YOU its fine.
As the word User Interface implies, it is YOUR interface
Hope i made myself clear.
One thing:
You have both bar display for target debuffs and aura debuffs on the frame, might only want to have the more convenient one.
Otherwise I think the rest is stylistic and placement, such as having a mixture of textures and borders makes the UI look a bit disorganized ( may want to look at having a general aesthetic to tighten it up). Also just the choice of add ons, such as if you do not spend time looking at recount graphs you may want to see if you like skada better.
TBH I agree with Zérenty, things I've mentioned or others may mention sometimes will be personal preferences so just see what works for you.
The best advice I can offer you is to look around at different addon-packs/complete UIs out there to see what you like and, maybe, even implement them into your own UI.
Inconsistent fonts, not least with your combat text.
I think a square buttonfacade skin would fit better than a round one.
Not a fan of big black bars across the botton, or more accurately described as a viewport, but thats a personal view.
I agree on the huge viewport at the bottom, it is just useless. It takes 1/5 of your screen.. If you insist to have background for chat, use chat background. Rest is obviously taken from Tukui,. it doesn't match your changes tho.
That would be my feeling about your UI ,)
I'd resize your WorldFrame and make its height less. From what I can see you're losing a lot of FoV from the bar at the bottom - this would change if you used WF resizing.
you could blend the recount title bar with a black/blue color
Skada has always given ME weird stats and stuff that often does not coincide with what i should be seeing. but that may just be me.
For ME what seems to be missing that i would have is some sort of threat meter. Omen3 is what i use. Also what might work better is to put the buffs you have relative to the side you have your character health bar and put the buffs relative to the side the enemy's health bar is.
I suppose that minimap, and those frames which display gold, friends and such are part of TukUI?
By weird stats, do you mean something which is different from recount ?
It's easier to give you hints if you post a pic while in raid, not just attacking a dummy