I voted someone in my family, which would be my sister. She is actually what is called "dual-diagnosis."
I'm undecided where issues like depression, ADHD, etc. fall in the world of mental illness. I believe that psychology in general is based on theories, opinions and experiments, not facts, and as a result, psychologists can and will have differing theories. I don't have much experience with depression so it's hard to say.
In general, I think that as a society, we focus too hard on finding a label so then it can be treated. Some people are quickly labeled as having depression and then are treated accordingly without really looking at what is going on.
Yes, I think depression is serious. But I also absolutely think that our society is too medicated.
Issues like schizophrenia, personality disorders, bi-polar....basically everything you learn about in any gen ed psychology class are in my book what is considered serious mental illness.
But this is just my theory.