The front hull has beefed up (applique?) armour. This alters the lines of the front of the tank, compared to a straight A5/A6 it appears "smooth" across the front instead of having a distinct separation between the hull and fenders/mudguards. (Look for photos of the Swedish STRV 122)
The HB A6EX kit also includes shrouded smoke grenade launchers and an APU in the right rear hull. (I believe the APU is not fitted to a straight A6) One of the less impressive aspects of the kit is the heavy anti-slip texturing, which one could mistake for access panels or applique armour.
Note that the 2A6EX is/was a one-off technology demonstrator vehicle built by the manufacturers to woo export sales (hence the "EX" tag) and didn't enter service. There are some which are similar, such as the aforementioned STRV 122 and the (Greek Army) Leopard 2 HEL (Hellenic), which incorporate some of the upgrades, including the uparmoured hull.