To add to this, one other category I wanted to emphasize is the lack of UI related features still. This includes things such as getting the color of a pixel at a specific point, being able to set the size to a negative value to invert/mirror an image (this would be especially useful for mirrors), and the inability to apply rendering effects to UI without a hacky solution (such as UI blur that can only be done via non-ScreenGui types of Gui, which requires much more effort than what should be required).
Quick points from above:
- Pixel color at screen point
- Invert/Mirror images/UI elements (via negative size) (Especially ViewportFrame)
- UI render effects (such as blur)