Хлебом не корми: someone's favorite thing
I think we can all agree that Russians can get pretty passionate about things — perhaps even more so than other nationalities, although I'm not sure how to go about researching that. There is no nuts-o-meter that would allow cross-cultural comparisons. But in any case, the Russian language offers a plethora of ways to express passionate interests.
To describe lovers of the arts, Russian began to use a word borrowed from French, меломан (music lover), and then generated балетоман (ballet lover) and киноман (cinema lover). A theater lover is described with the homegrown terms театрал for a man and театралка for a woman. For those whose passion has crossed the line into addiction, Russians have borrowed from English to describe алкоголик (alcoholic), трудоголик (workaholic), and even шопоголик (shopaholic).
Or you can just call someone любитель (lover) and add a noun. A friend with a great appetite calls himself любитель вкусненького и неполезного (a great lover of everything that tastes good and is bad for me). Sometimes you can replace the noun with a gesture. The same friend adds: И я большой любитель этого дела (And I'm a great lover of doing this) and snaps his fingers on this side of his neck to show what "this" is: hitting the bottle.
The only tricky bit about the word любитель is that it can also mean an amateur: В труппе есть профессиональные актёры и режиссёры, и есть любители (The troupe has professional actors and directors as well as amateurs). When любитель is combined with an art, object or sport, it doesn't mean a lover; it means a nonprofessional. So кинолюбитель is an amateur film maker, and автолюбитель is a nonprofessional driver.
If a person's love for something is ramped up high but hasn't quite crossed over into addiction, you can call him фанат (fan, fanatic). This is most commonly used for sports fans: Толпы фанатов требовали автографа у футболиста, когда он появлялся на улице (Crowds of fans asked for the soccer player's autograph whenever he went outside.) But it can also be used to describe anyone whose passion for something is a bit unhealthy: Отец был фанатом своей профессии и мог стать жертвой зацикленности (My father was a fanatic about his work to the point of obsession).
If you want to describe a less frenetic passion, you can use the word почитатель (admirer). This is a good word to pull out of your linguistic pocket when you unexpectedly crash into an author at a book store reading: Я являюсь вашим давним почитателем (I've long been an admirer of your work). Just hope the pleased writer doesn't ask: А какая моя книга больше всего вам нравится? (Which of my books do you like best?)
I like the slightly old-fashioned word падкий, which describes having a weakness for something or someone. The word is either followed by на and the accusative case or до and the genitive case. Он падок на лесть (He's a sucker for flattery). Она падка до сладкого (She's got a weakness for sweets).
Another vivid expression is хлебом не корми, which literally means "you don't even have to feed him bread." The idea is that someone loves to do something so much that he'd give up food for it. Вас хлебом не корми, только дай поболтать, причём не по теме. (You like nothing better than to talk — and especially off-topic).
In other words, he's любитель поговорить (a real talker).