Interesting discussion! For me it would have to be:
1. Animal welfare in the broadest sense. Animals need to be happy and healthy, but I also like to see interesting enrichments, a green (or natural looking suited to the species) environment, and I think a place where animals can hide themselves from visitors during opening hours is an essential as well. A great plus for me as well is staff monitoring in walkthrough exibits.
2. Interesting mixed-species exhibits, I love seeing special interactions between different species of animals. I like the common safari combinations as much as any other person, but mixing other animals living together in the wild is interesting to see. Taking point 1 into account of course. I also like it when a zoo blends in the regional nature in their zoo or exhibits. (My home zoo just implemented breeding spaces for local snakes, recycling elephant manure.

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3. On the subject of visitor experience: I like it when a zoo is green and natural looking. It makes me feel like I am blending into the exhibits and feel more part of the zoo experience (if this makes any sense to anyone.) I also like when I am not obligated to follow one set out route. After my first walkaround I like to hop around and visit species that were hiding when I came by, or just my general favourites. I also like finding the quiet spots of the zoo and getting away from all the humans at some point.

And a great plus for me is enough restroom and coffee-facilities.
4. Being surprised by something! A less common species, interesting enrichments, an interesting viewing point, learning something on animal conservation or nature I didn't know yet, or learning something about how the zoo contributes to scientific research. (Brownie points for carrying a book on the zoo's history or scientific research.

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5. Elephants! Honestly, not seeing my very favourite animals makes me a bit sad

In the best case with a matriarchal family group and more than one youngster.