What single noun denotes a person who is interested in something?

enthusiast (ODO)

noun
1 A person who is very interested in a particular activity or subject:
a sports car enthusiast
syn: fan, … supporter, follower; … informal buff, …

Cited on TFD:

He was as much of an enthusiast in art as I was, and not less anxious to learn to paint. A Tramp Abroad by Twain, Mark

Dilettante

dil·et·tante noun \ˈdi-lə-ˌtänt, -ˌtant; ˌdi-lə-ˈ\ : a person whose interest in an art or in an area of knowledge is not very deep or serious

http://www.merriam-webster.com/dictionary/dilettante

Unfortunately, once you get into these words such as amateur and layman that are less-serious, you do tend to have negative connotations. "Dabbles in..." is a construction I'm fond of, and I was planning to suggest "aficionado" but, as it turns out, that has a more serious denotation than I expected.