My biggest peeve is when maps use some shit version of highway shields to sign numbered routes. I'm look straight at you, Delorme. When they started switching over their cartography to the current style a decade or whatever ago, their interstate and US shields are just dogshit. What the hell is that even?
They would also commit a cardinal sin of using the same line style for business interstates as regular interstate highways. Yuck!
I have this recreation atlas in my vehicle of southern Wisconsin that attempts to show public lands and trails, but boy do they phone it in sometimes. It's like if they didn't have the actual boundary of a park, they'd just fill in all the space enclosed by the nearest road. But that's not as bad as using the same line style for a ramp that they do for the freeway (or whatever). It looks terrible, cheap, and lazy, man.
Oh labeling unsigned routes on the map; that helps no one. RandMac is guilty of this for all those 'follower' state routes that shadow every US route in several southeast states. Unnecessary clutter. Get rid of it.
Failure to indicate non-interstate freeways as such; the first offending example that comes to mind in the the official MnDOT map of Minnesota. I hate that style.
Oh and speaking of Minnesota's map, unnecessary detail is bad. Like how MnDOT tries to show the configuration of every interchange on the city insets. On one level I get it, but it's difficult to make that look good at that scale so it's best to not bother. At a previous gig I sometimes had to make maps where the customer wanted unnecessary detail; every road at a scale where it is not appropriate; that kind of thing. And it looks bad. It's like someone sneezed on the map but their boogers were all letters.
If a map is using squares for interchange symbols, they look so much better when they are oriented to match the angle of the roads that are intersecting at that interchange.
In general I hate maps that look like they were clearly auto-generated by a GIS application with very little review by human eyes. Algorithms should only be a starting point for the final product, not a cheap-ass shortcut to churn out any ol' piece of shit.