IMO the graphics are superior to the 1st.
For a start its using Cryengine 3, which I doubt they would make to be worse than the previous Cryengine 2.
Crysis 1 was hugely unoptimised, and took a huge amount of computing power to run at high settings well. Warhead aimed to fix some of this, which it did, but it was never perfected.
You fail to see that DX11 would not make much difference at all. The first one wasnt running DX11..
The main advantage is its implementation of larger scale Tessellation, and I highly doubt they would cram the game full of it as it would up the required system specs, and just isnt a viable option at this moment in time.
They didnt use DX11 in this game as consoles are still running DX9 hardware, this is not a viable option as its just a port.