Dallas Zoo sure does "knock it out of the park" in terms of displaying African wildlife. From the magnificent Giants of the Savanna complex to the wonderful great ape habitats (gorillas and chimps) to the monorail journey through various biomes and finally to the walking section...what a truly great set of African exhibits! Not many zoos on the planet have such a well-designed set of enclosures to showcase the finest that the dark continent has to offer.
I agree, and Los Angeles Zoo has a similarly sloping chimp exhibit that also has a rocky section and a better backdrop. Of course, I'm not sure that anything can top Kansas City's 3-acre chimp paradise.
To have put in a moat would have reduced the possibility of other exhibits in Africa. The area isn't really so large and moats take up a lot of space.
Rockwork is, of course, a budget issue.
A sensitive painting of the wall might be the most practical improvement.