The Lip Line
I would say it's the angle of the swell compared to the angle of the shallow sand bank or reef that it is hitting.
If the wave hits the reef more square and straight on the lip line will fall much like the WOTD, and be a real fast racey wave.
If the wave say comes from 45 degrees onto the reef it will break more like the photo in the 'Real Experience - Part 2' thumb thread as the rest of the wall of the wave is travelling slower as it bends in and around onto the reef.
There are obviously other factors but for the most part the faster the wave is peeling the more it will look like the WOTD, and the slower the wave, the more it looks like the Session Thumb.
Just wondering what determines the 'lip line' in a wave? Is it the shape of the reef, angle of the swell, direction of the wind or combination of all three?
If you look at today's WOTD the with Mick Marjanovic the 'lip line' stretches way back. On 'The Real Experience - Part 2' image the lip line is more straight up and down.