Combine cabbage with lime juice, cilantro and 1/2 teaspoon salt. Mix well and set aside.
Heat large, heavy skillet over high heat (or heat electric griddle to 400° F). Season both sides of steak with salt and chipotle powder. Drizzle with olive oil. Place in hot skillet; grill 6 minutes per side. Remove from skillet; let rest 5 minutes; then cut against the grain into 16 strips.
Wipe skillet clean; return to heat and warm tortillas a few seconds to soften. Fold to make taco shape; fill each taco with 2 pieces of steak and 1 to 2 strips of avocado. Top with cabbage and sprinkles of feta cheese.