1) Satisfying. Each meal (or snack) should be able to keep you from feeling hungry for as long as you need it too--anywhere from three to five hours. The frequency with which you eat will depend on each person. There is an argument to be made that eating more often keeps the metabolism revved up-- and that will help with weight loss. It's an argument. Some can do this. Some want to and can eat more often than others, for others, (like me) this just leads to inappropriate eating. I prefer to eat as I did on the whole 30-- just three filling meals per day, no snacks. To my way of thinking, the less I need to eat, the less I will actually eat, and the less I prompt a rush of insulin into my bloodstream, the better.
2) Adequate calories for activity. (Slightly fewer for weight loss.)
3) Lots of vitamins and minerals (and other stuff science is only beginning to identify) found in whole foods.
4) Nothing you are intolerant towards, whether that be fat (my gallstones) or gluten (celiac disease) or lactose, for example.
6) Sustainable. This is more than just affordability, it also includes variety (if you can afford that), and whatever it requires of you to prepare the food you're eating. Some advocates of paleo diets advised their participants to "cook on the weekends" and if they meant cook all weekend (or even one day) for the week ahead, there's no way that would work for me. Not right now, anyway. For others, that might be fine, even fun.
I've left out the whole macro breakdown on purpose. (That is, what proportions of carbs/fats/protein should be in the diet). Human beings have survived on all sorts of diets--and no particular macro breakdown is appreciably better than another for weight loss.
Are there other things I should be considering? Is there anything I've left out?