I mostly agree with what Otter Lord says.
As far as the most sunny days comment, how about the most sunny days that people would actually want to be outside in.
Anyway, concerning the pace of improvement...
What was good about the SDZ before 1980 other than the inherent advantages of being in San Diego?
Haven't the vast majority of the exhibits that make the zoo considered among the best been completed since then?
Pre-1980 features of the Zoo that were/are noteworthy include the two giant walk through aviaries, the skyride, several of the open-air reptile enclosures, the former "Cascade Canyon," the Birds of Prey aviaries, the moving sidewalks, the hummingbird aviary, the siamang and gibbon islands, Fern Canyon and the now-closed Goat and Sheep Canyon.