The following thing I find infuriating in the rulesbooks for board games:
Multiple Gameplay Walkthroughs- Having more than 1 gameplay runthrough, with the rules split between them, makes it extremely hard to look-up rules. It also confuses people as they are trying to go through the rules to learn the game. Have all the rules in one spot in a logical order. See Portal Games for the worst examples.
Missing Significant Rules- If something is in the game, it should be defined in the rulebook. This is a problem with Kickstarters, with terms defined (oftentimes not intuitively) on cards that are just missing from the rulebook. See Endure the Stars (1st Edition).
No Player Aids or Rules Only on Summary- The summary should give you the rules that have been explained in depth elsewhere. If your game has 11 different actions (with effects that aren’t indicated anywhere) and an action only fully defined on a summary, you need a decent player aid and blind test. See In The Name of Odin.
Wall of Text- This is self-explanatory. Things should be set out with headings, examples, etc. Having just a wall of text to try to find rules and definitions is a nightmare. See Ravens of Thrh-Sahari.