Mostly referring here to the screenshots on your Generic UI download page.
Calligraphy font is basically unreadable.
Other people already pointed this out, but it's bad enough to point out again. While it might look "cool" in screenshots, it's not very practical for quickly reading text in the middle of a fight.
This is especially important for a UI you're releasing for other people to use. Most people I know who are under the age of 25 can only read cursive at the same speed as a 2-year-old puzzling out the sounds of words in a "learn to read" book, because cursive is (apparently) no longer taught in schools here in the US.
Such a hard-to-read font will likely turn off many people who would otherwise use your UI. Use a clean font with distinct letter forms and little to no decorative elements.
Also, make sure that text has enough contrast with the background, or has an outline. Your damage meter is completely illegible with white text on bright class-color bars.
Different bar textures everywhere.
Pick one, and use it everywhere.
I'd suggest
not using the one from your player/target frames, as the left side of it looks horrible with the texture stretched to a unit frame width.
Different borders everywhere.
Pick one, and use it everywhere. Small inconsistencies like these add up and make your overall UI look sloppy and unorganized.
Minimap needs a border.
Currently it's hard to tell where the minimap ends and the game world begins.
Nothing is aligned with anything else, and some things are even overlapping.- The top of your chat frame should line up with the top of the action bar next to it.
- The left side of your player frame should line up with the left side of the action bar below it.
- The right side of your target frame should line up with the right side of the action bar below it.
- The spacing between the player frame vs. action bar is less than the spacing between two buttons on the same action bar. It should either have the same spacing, or larger spacing since it's a different UI element (preferrably with an even multiple, such as 2x spacing).
- The top of the minimap is flush against the top edge of the screen, but the top edge of your group frames is not.
- Boss timers are overlapping the boss frame.
- Dungeon size/difficulty icon on the minimap overlaps the clock.