Thalassemia Intermedia is not just "a little" more serious than minor. It can have a lot of differing symptoms with varying levels of symptoms and severity. You might want to ask the Thal Intermedia board for a generalized answer...Also, cooleysanemia.org has a great description of general symptoms.
Also, Thal minor can be symptomatic or asymptomatic, which is called Thalassemia Minima, or Thal trait. There is a wide vareity of thal minor symptoms, and they are doing research and finding out more everyday.