There is a rumor floating around the philatelic world that the American Topical Association is going to be changing it's name to include the word "thematic." In the U.K. and other places in the world topical collecting is known as thematic collecting.
There is already an Indian Thematic Society, a thriving organization, as well as an affiliated chapter to the ATA located in Ludhiana, India.
Topical or thematic (you say po-tay-to, I say po-tah-to) gains more active collectors as the specter of the impossibility of putting together a complete country collection grows with every year's flood of new issues.
From my own experience (Lighter-Than-Air philately, with a specialization in balloon posts) I can attest to the endless fascination involved in putting together a good topical collection. Without the pressure of having to buy a country's most expensive items -- including rarities -- topical collecting can be pure enjoyment.
Meanwhile, if any reader can verify or quash the rumor, please feel free to do so in the comments below.