A few things on top of extra workbench screen selections. It is ECS Agnus.
The Super Denise supports programmable scan rates so it can generate 31 KHz screen modes without any extra hardware. Unfortunately, the palette is very limited in these modes. This also allows audio playback rates > 28 KHz, superhires (1280 pixel horizontal, max 4 colors out of only 64).
New genlock features. (new ways to choose "transparent" color)
Improved support with non-PAL/NTSC (programmed) modes (display window size/position etc..)
Fixes at least one denise bug related to strobe register handling, this causes blank screen with some demos if super denise is installed.