Apart from the obvious answer that it originates from India, a more concrete answer to the above question is quite complicated but very interesting.
Indian history is extraordinary, shaped by numerous unique cultures that resulted from invasion, migration and merging of different tribes. This reflects in its unique cuisine. While everyone can find something unique about Indian food, be it the curry which is a complex but unique blend of spices or masala or the exotic looking lamb and chicken dishes, any attempt to pin it down has been futile so far.
Take the humble chutney. This simple blend of fruit and spices could not have originated anywhere else around the globe but India. Who else would dare to be so inventive as to mix a sweet, in this case, ripe fruit, with a blend of spices?
While chicken is a common ingredient in almost all types of cuisines, no chicken dish is internationally acclaimed as the Indian Tandoori Chicken. The sizzling clay oven in which it is cooked and the vivid colors of this dish is only a part of its unique taste. The perfect blend of spices and distinctive method of preparation is what makes this dish an international favorite.
Similarly, lamb is an integral part of Italian, Greek and a number of other cuisines. These places also boast of rich history and culture, but the flavors of Indian Achari dish is something exceptional when compared to, say a Greek lamb stew.
Is it the coriander? But then it is also found in Greek cooking. Likewise, tomatoes are a common ingredient in many cuisines. Can it be the exotic Indian spices that give the uniqueness to Indian food? Well, Greeks were renowned for their travels worldwide and brought home many spices from far off exotic places. The discussion can go on and on, but the fact remains that Achari is a pungent, exotic dish that is a treat to your taste buds, while the Greek stew is tasty but mild.
The doughnut which is an oil-fried, fat-filled, sweet dish is omnipresent in America as well as in China. But the Indian vada which in no way resembles a doughnut is crisp, savory and flavorful.
While you can try to pin point the uniqueness of Indian food with its use of spices or colorful presentation, the exact answer is still a mystery, just like the country it originated from.