I've met a bunch of lovely Haskell programmers, but I'm always surprised by the range of PL interests they have.
(1) Some are particularly into lazy evaluation. I get it: you can do some really elegant code for some problems. I've enjoyed this in other PLs too.
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It's odd how lazy evaluation is generally seen as a niche design choice, yet the vast majority of languages treat `foo() || bar()` as short-circuiting.
It's really satisfying to use a profiler for the first time on a project. I always find a big performance win with only a small code change.
It's never the code that I expected to be slow, however!
I've started keeping a list of particularly interesting bugs and patches that I've worked on: https://github.com/Wilfred/interesting-code
The time that I once removed *a single closing paren* in Emacs is still my favourite.