In a small bowl combine the brown sugar, chili powder, paprika, garlic powder, cumin, salt, coriander and cayenne. Rub the olive oil on both sides of the steak, then rub the spice mixture over the steak and set aside.
Preheat the grill to medium high. Grill the corn, turning as needed, until the kernels are lightly browned on all sides, about 6 minutes. Cool the corn, then slice the kernels off the cobs and place in a medium bowl.
Grill the steak for about 4 minutes on each side, until nicely marked with grill marks, and cooked to your liking. Transfer to a cutting board and let sit for 5 to 10 minutes for the steak to reabsorb its juices. Thinly slice the meat across the grain.
While the steak sits, add the tomatoes and scallions to the bowl with the corn. Drizzle over the olive oil, lime juice, and season with salt and pepper. Pour over the salad, and toss. Just before serving add the avocado and cilantro and toss again.