My taste is split between how I like digital rulebooks and physical ones. Digital ones I like a minimal amount of graphics and a clear layout that makes sense without colour because I have to go get the things printed for use in games and that can double or triple the price to bring a set of rules to the table. For rulebooks that are available in physical format, I like them to take advantage of that and make full use of colour and graphics but with the important distinct of not becoming cluttered and difficult to read. Tomorrow’s War is a well known example of a system marred by poor layout and being made very difficult to process for a lot of people’s eyes due to the backgrounds.
What I really like though is down to how a system is written. I want to be able to reference the main book whilst in play with speed. I want to be able to find the correct mechanics and have them clearly explained, and formatted so that they’re easily findable and the way they are written should be easy to grasp the relevant bits. Good examples of this would be A Fistful of TOWs where every chapter has an executive summary with the bullet-pointed list of the basics followed by well spaced out decently large text too in a readable font. Thought FFoT is helped by the fact that the rulebook was so large that making it in colour with images would have been insanely costly . A poor example would be Pike & Shotte where the conversational style of writing means relevant rules are buried in long columns of small text with frequently ambiguous descriptions. I also found that All Quiet On The Martian Front suffered from being over-produced, with fluff and images seeping into the mechanics sections and things being just badly organised to the point that a multi-page fan-made quick reference sheet was needed to play a basic learning game.
I guess the TL;DR is that I don’t like it when a system’s ‘large’ production value gets in the way of being able to learn and play it. It’s a tricky balancing act and I’ve seen repeatedly that production value can win interest far more readily than rules ever can, so it’s entirely understandable that it can get to the point of being in the way.