A post on MyCrisis, hastily taken down shortly afterwards, reveals some details about the update, along with a screenshot. Thankfully NeoGaf user BeeDog managed to grab it before the takedown.
UPDATE: Vivec from the forums managed to grab the side by side comparisons before the site went down, take a look inside.
The text of the page reads as follows:
DirectX 11 Ultra Upgrade
The DX11 Ultra Upgrade is a free visual improvement add-on introducing DirectX 11 support as well as a wealth of graphical improvements and performance optimisation for both DX11 and DX9 APIs.
When using the new 'Ultra' spec, DX9 platforms will benefit from realtime local reflections and contact shadows. The owners of DX11 platforms , in addition, will be able to enjoy hardware tessellation (requires the installation of the "Tessellation Package"), parallax occlusion mapping and several improvements for shadows, water, particles, depth of field and motion blur.
DX11 Tessellation Pack (optional download)
This free pack containing pre-tessellated geometries is necessary in order to be able to enjoy hardware tessellation on DirectX 11 graphics cards. A compatible DX 11 graphics card is required.
High Res Textures (optional download)
This high resolution textures package provides higher textures resolution for a wide variety of assets. It supports both DirectX 11 and DirectX 9 versions of Crysis 2. A 64 bit operating system and graphics card with at least 1GB of memory are required.
Crysis 2 is already an impressive looking game, so the promise of "parallax occlusion mapping" is just icing on the cake.
Update - Side by side comparisons grabbed by Vivec