This is drifting the topic slightly, but it feels relevant with how much subjectivity is bleeding into the discussion. Speaking for me personally, I hate rushing a zoo. Friends and family that accompany me are often impatient when I stop to view every terrarium in a reptile building or choose to, say, take a seat outside a large exhibit, or stop to focus on a really active animal - it's part of why I've swung in the last two years to being more comfortable going on these trips alone, because then I can afford to take my time, and if I miss something it's my own responsibility.
I've been trying to plan my trips around having two opportunities to visit a given zoo in case anything a second chance is needed; this is not just about two-day zoos, but also having the chance to drop back in for anything I might just want to experience again or if I miss a rarity. I had such a great time at Denver last night and I've been desperate to go back; when I visited the National Zoo years ago, me and my family were too late to see the pandas, so we came back the next day to see them.
I've done two two-hour trips to Lincoln Park by skipping African Journey, Walter Family Arctic Tundra, Penguin Cove, Pritzker Family Children's Zoo, Camel-Zebra Area, and Farm-in-the-Zoo, which skips over half the zoo's total land area; but this is my local zoo, I can go there 'any' day, so next time can be in a week if I feel like it. It's a nice luxury to have.
I completely understand why anyone might have to rush because they are strapped for time, and some tickets might be pricey to pay twice in a week, but I still want to say I pity anyone who makes a zoo visit and has glance at each exhibit for a few seconds and then move on in order to fit everything they want to do. I personally love having the option a few times during the day to sit by an exhibit for a little while and really take in the animal's behaviors, or sometimes wait for a glance at something I'd have missed in a rush, like the mandrill and aye-aye at Denver.