My interests are odd (for a computer geek). I don't think I'm retro, but I've learned Ruby, Lua, Erlang, ML, OCaml and a bit of haskell in the the past 7-8 years (yep, I first picked up on Ruby in 2000). I really haven't learned a new language (or done any real programming in the above languages) in the past 4 years.
I'm interested in [La]TeX again. A couple of posts ago I mentioned working on AFT. That could be fun, but (if I had the CFT) I'd rather hack around with TeX. In particular, I am interested in learning MetaPost (look it up). There is a nice $50 LaTeX graphics book that covers MetaPost.
So, I am thinking to myself, wouldn't it be fun to do some hardcore typesetting? (Remember, my interests are odd.)
Fundamentally, I love the idea of TeX (and LaTeX). The end result is very tangible: A (potentially) beautiful typeset document. Maybe make a book out of my kids' artwork or something. TeX (and LaTeX) is a complex enough system that it would be fun just to get lost hacking in it.
Emacs is a large hackable system too, but the end result is some editing tools.
Unix is a large hackable system, but the end result is doing some process/text manipulation.
With TeX, the end result is a piece of paper. A well typeset piece of paper.