Let's start with a weight comparison! However, it must be noted that this weight comparison is not that accurate, as the Ultegra 6800 11 speed crankset is a compact 50/34T crankset, while the Ultegra 6700 10 speed crankset is a standard 53/39T crankset. Both have 170mm crank arms.
Ultegra 6800 complete crankset, 677 grams.
Ultegra 6700 complete crankset, exactly 700 grams.
Left crank of Ultegra 6800, 197 grams.
Left crank of Ultegra 6700, 201 grams. Just a tiny bit heavier.
Right crank + chain rings of Ultegra 6800, 480 grams.
Right crank + chain rings of Ultegra 6700, 499 grams. The weight difference is probably due to the larger chainrings.
It can be seen that the Ultegra 6700 crankset is about 23 grams heavier than the Ultegra 6800 crankset. This is a relatively small difference, and is probably due to the larger chainrings on the Ultegra 6700 crankset.
Now, let's take a closer look at the left crank arm. As shown below, the Ultegra logo direction has been changed between Ultegra 6700 and 6800. On the Ultegra 6700 left crank arm, the Ultegra logo is right side up when the crank arm points backwards. However, for Ultegra 6800, the logo direction has been changed such that the logo is right side up when the crank arm points forward. The new Ultegra logo is also larger.
Difference in logo direction between the Ultegra 6700 left crankarm on top and the new Ultegra 6800 crank arm at the bottom.
The new 6800 crank arm on the right is generally wider than the previous 6700 crank arm, and yet is more lightweight.
New 6800 crank arm on the right is manufactured by bonding 2 halves of the crank arm together, as can be seen from the seam on the crank arm. Previous 6700 crank arm was forged in one piece.
Ultegra 6700 crankset on the left, new Ultegra 6800 crankset on the right. Visible difference in chain ring sizes. The Ultegra 6700 right crank arm is 5 arm and forged in one piece, while the Ultegra 6800 right crank arm is bonded with 2 pieces and is a 4 arm design.
Overall view of the 2 Ultegra cranksets. Quite different appearance, as the previous 5 arm design has been replaced by the new generation of 4 arm crank design.
1) Previous 5 arm design vs new 4 arm design
2) 2 piece forged + bonded construction for the new 6800 crankset, vs 1 piece forged construction for the previous 6700 crankset
3) Logo direction on the left crank arm is different
1) Similar glossy grey colour and surface finishing
2) Hollowglide outer chainring
3) Similar weight (accounting for the different chain ring sizes)
4) Same hollow spindle and crank arm fixing method
The main difference would have to be the appearance, from the 5 arm design to the 4 arm design. Otherwise, the construction and the surface finishing are quite similar.