A Good example would be the colobus, who have garnered such crowds you'd think you were lining up to see the black and white fur of a panda and not of a fairly common monkey. .
As you said further down, the secret is in the exhibition-style. The new walkthrough for the Colobus at ZSL is different and interesting for the public, so they'll queue just to see a fairly common monkey that is exhibited at many other UK zoos without any fanfare at all. I also think once inside a zoo, visitors are drawn to certain species, mainly whatever high-profile species are available at that particular zoo, which are not necessarily their own favourite animals- if you get my drift.