Pretty much everything visible has had a change, but the short gist of it;

• Adapted UI to resize itself depending on user resolution. See supported resolutions thread for more info.
• Most windows have increased height and width, to increase space for relevant content.
• Increased the height of most list elements.
• Fixes and optimizes vanilla UI wherever possible.
• Bigger Empire Creation.
• New Planet Viewer (See below for info).
• New Ship Designer that vastly increases the whole window space to allow for more ship sections, like New Ship Classes.
• New Fleet Manager that hopefully increases effectiveness: Image
• In-game usable Tech Tree, made by: turanar. (Using the built-in web browser)

You can also see the changes in the previews up above.

The planet viewer was recently changed, and as I know people hate change, there are alternatives:

• Bigger Planet View (Will retain the old design)
• Bigger Planet View - Building Box Dirty Fix
(If you want more than 36 buildings slots)

• Planetary Diversity - Planet View (Vanilla look, with larger cityspace)

Use only one of them.

I'd recommend putting this mod at the very bottom, as in last in your load order.
Only exception is with specific instructions given with compatibility patches, or if you want a specific UI mod to overrule the changes in this one.