I really like mixed species enclosures, I think they make an exhibit a lot more interesting- and also from a zoo point-of-view allows for better use of space. I like to see mammals and birds displayed alongside each other because it is quite unusual.
However, like Pygathrix suggests its not enough for the animals just to be able to survive side-by-side, but actually be happy. It seems unusual and a bit unfair to mix carnivorous animals, even if they don't pose a real threat to the animals they are sharing with. I saw a photo from Kiang, of Highland Wildlife Park, with foxes sharing with Snowy Owl. I don't think that that's exactly a recipe for success. Similarly, I'm not too convinced on the zoos that keep brown bears with wolves. I have read bits about South Lakes creating rather ridiculous and less-than-successful mixed species.
I also found it quite annoying that Bristol Zoo, amongst others, mixes species that have no overlapping range in the wild. An example would be keeping Eider Ducks and Inca Terns with African Black-footed Penguins.
Some mixed species enclosures I do like are Chester's Europe-on-the-Edge Aviary (though I wonder whether all its occupants are happy about sharing with vultures?), Edinburgh's African Aviary with Dik-Dik in, London's Clore Pavilion, Longleat's Monkey area where they share with Blackbuck and Water Buffalo and Bristol's Sugar Gliders and Potoroo.
I have some questions about mixed species exhibits actually. Does anyone know of...
Zoos that house penguins with other species?
I know about Bristols and that Chester used to keep theirs with black-necked swans. Does anywhere keep humboldt's with Mara? I think there's a zoo in the US, which keeps its penguins in a big walkthrough aviary, but at the minute I can't remember its name.
Hippos in a mixed species exhibit?
I wondered if anywhere kept Nile Hippopotamus with anything like pelicans, or lechwe, reedbuck, waterbuck and the likes. I think that could make a good exhibit.
Elephants in mixed species exhibits?
I seem to think there is or was a Swedish zoo keeping African elephants alongside ostriches and zebra. Has anyone else heard of that? What about Asians, am I right in thinking Edinburgh may've speculated about keeping Asian elephants with gaur?
Nocturnal mixed species enclosures?
Would it be possible to keep Douricouli with a South American bat species?