This question really piqued my interest, because it made me realize that the way my PC is dictates how I've built it and how I'll grow it. It's kind of eclectic, as it's on:
5 Players, of whom I collect their RCs, on-card autos and inserts: Koufax, Kershaw, Robinson, and Evan Gattis
1 Team: (Go) Dodgers
Contemporary RCs and key RCs of the 80's - 90's
Several umbrella topics: cards with stamps, Topps Pie in the Face, U.S. History, Golden Age Hollywood, assorted odd stuff
Completed base + 1 subset of each year's Allen & Ginter
Tristar Obak: looking to complete 2009 and 2011
Because I am not made of money, I can't buy boxes of everything that includes stuff I'm interested in. So I've realized that I've needed to trade for it, which I only do here, and only certain times of year, when my budgets for free time and money allow - which made me realize that I should interact more here to make more trading friends!
Every so often I'll snag something from the Bay. There is only one LCS near me, which I've visited with mixed results. I buy 2 boxes of A&G per year (maybe this year I'll be able to afford three, unless that feels like overkill). Maybe one day I'll check out a show.