I must say, my experience of San Diego zoo is similarly mixed.
The zoos strong point was extremely large animal collection. And the San Diego climate is wonderful. Typically for American zoos, many exhibits were small but the staff was super-helpful and giving you all the attention to answer your questions.
However many exhibits were underwhelming - giraffe, elephants and lions looked like a zoo from 50 years ago. Oddly, some new exhibits were much worse than old ones - for example the infamous elephant odyssey compared against apes, tigers or giant aviaries. The problem was also lack of any information what species the zoo actually holds and where. For me, I missed a small row of aviaries with the only Tongan megapode in human care, a species I am unlikely to ever see again. But other people miss ABC animals. I cannot understand why a commercial zoo for years made no effort to sign its aviary nor keep the animal list online - it should be very easy. Perhaps part of the problem is extreme entry price and hype which set the expectations very high.
I agree with both points, the San Diego's collection is large and vast, and the weather is nice even though it was cloudy the day I went, which sort of gave off a rather depressing mood that day. I didn't get to talk to staff aside from the one who told me about the honey badger being exclusive to the bus. I was unfortunate in that such a large portion of the animals were off exhibit the day I went - the tasmanian devils, harpy eagles, African eagle, gharial, pronghorns, hamadryas baboons, bonobos, only one little orangutan in their exhibit, absence of male gorillas, anteater, jaguar, fishing cat, flamingos, fossas, binturong, a lot off display that day, making me wonder how usual of an occurrence that is. I've never seen so many mammals off display at once, not to mention the penguins in their temporary enclosure, the okapis temporarily at the Safari Park, and the hippo exhibit under reconstruction.
The lack of info on what species the zoo holds and where got to me too, as the site lists both their zoo and Safari Park inhabitants all on one page. Many animals absent from the zoo are listed like the wombat, hyenas, etc. I was disappointed to not see the tasmanian devil and am sure I might not ever see them again, what with their short life spans and rare distribution. It would be nice if their site was updated. And I understand that not every animal can have the best exhibit in the world, especially for smaller critters, but I too was caught off guard by the ABC animals, being their popularity I would have assumed that they'd at least have really nice, large, and aesthetically pleasing enclosures, especially the lions. The price and hype definitely set my expectations high, it was not a bad zoo by any means, most of the animals aside from the bears and penguins did not look stressed or unhappy which is the most important thing. But I expected more considering how hyped the zoo is online. Google "best zoo in the world" and it is literally number one on every one of the first page's sites, articles, and blog posts that come up.