I've recently upgraded from version 0.9.4.5 with Columns UI to 0.9.5 with the Default UI. Compared to Columns UI I'm missing some features that I would like to see in the new Default UI:

Inline Tag Editing
Columns UI allows to edit tags directly in the playlist window by clicking two times into a field, similar to renaming files in the Windows Explorer. Additionally it would be nice to also be able to edit tags in a "Selection Properties" frame. I find that much more convenient for simple and quick tag editing than to open a separate properties window. Sometimes it's just to correct a typo.

Custom Button Graphics
It would certainly be nice to be able to replace the default button graphics with custom ones.

Custom Buttons
I'd like to be able to add new buttons (text or graphic) that can be bound to context menu (or other) functions.

Sorting Definitions
Columns UI allows to define how rows are sorted irrespective of the displayed field content. Sorting by certain fields doesn't always yield desirable results (example: ReplayGain values) if they are sorted without any kind of preprocessing.

i'm missing something too: activate playlist manager by single click; activate Ui elements by mouseover

It has already been said that this won't happen as it violates Windows UI standards.

didn't 0.9.4 violate the standards then?

The 0.9.4 default UI did not have any UI elements.