Less than one month til X and Y!
– Pokemon X/Y Pokedex
While its design changes don’t feel as drastic as Charizard or Blastoise at first glance, when making this wallpaper, I realized that there are lots of little details that are changed when Venusaur mega evolves. Aside from the obvious markings and the flower on its forehead, its plant got taller, having 3 branches instead of 1, and there are two tiers of leaves on the plant. The design of the flower on top changed slightly, and its body became more stout.
Mega Venusaur has the ability Thick Fat, which makes it take neutral damage to probably its biggest threats of Fire and Ice. In fact, a Grass type Pokemon probably benefits the most from Thick Fat. It would make Mega Venusaur an even better wall/staller.