Inspired by Phil Karlton's (possibly apocryphal) Two Hard Things, here's what I struggle with on a regular basis:

1. Getting information X from system A to system B without violating the N layers of abstraction in between.
2. Naming X such that the name is concise, unique (i.e. greppable) and has the right connotations to someone new to the codebase (or myself, a year later).