Very different ideas of what and how to cook, where and how to store foods (especially, what goes in the fridge and what doesn't), and so on. Instead of one kitchen with both our stuff in it and little shared, we could have much more orderly separate ones with only a little duplication.

It'd be fine if only one of us ever cooked, and if either of us didn't find the other's methods and tools frustrating to use at best, but that's not the case, so here we are.

Examples: my ideal kitchen would contain a rice cooker and zero crock pots. Hers would have zero rice cookers and at least two crock pots. Mine would have at most two non-stick pans of any kind—really, I could live with just one—but several cast iron pieces, and some steel. Hers would have exclusively non-stick. I'd be entirely content with a single short built-in oven to save space. She'd want a full-size double oven. That sort of thing.

[EDIT] also, whether it's OK to leave any small appliances on the counter full-time or they must all go in cabinets when not in use. We finally, after years, compromised on the stand mixer and we're up to a 4-slice toaster now so it's just too damn big to store, but the rest...