Besides looking for grain-free and real meat, some people look for gum and starch and gluten free due to concerns about those ingredients. Others avoid menadione and other ingredients.
I learned a lot from this e-book. It lists good/bad ingredients, explains label terminology so you understand what you are buying and has a lot of chapters on the pet food industry as well as the author's "approved" brand list, which is where I started from buying canned food and then once I understood what to look for expanded to include other brands:
The basic premise is meat in the first 5 ingredients, and as few fillers as possible. there are a lot of other ingredients that are on people's list as a hard no, and mostly we pick and choose what we want to avoid based on our feelings towards it and our individual cat's preference and health.
It's kind of personal choice. Corn, wheat and soy are deal breakers for me. I look for meat (even if it's by-products) as the 1st ingredient. I look for low CARB, not necessarily grain free. A lot of grain free foods just use a ton of peas or potatoes in the place of rice and such. Not any better. I don't mind by-products, gums or carrageenan.
I feed my cat canned Friskies and while many people seem to consider it a "bad" food, I don't see anything wrong with it. It has rice but it's such a small amount. It is lower carb than some high-end "grain free" foods. It has by-products but it's MEAT and animal protein which cats needs so...
For canned food, lately I have been buying my cats Friskies and Fancy Feast Pate, and they seem to do better on it than the gravy ones. For dry food I am still feeding the cheaper stuff, but Tractor Supply has some healthier ones that I might switch to.