I don't want to state the obvious, but of course signage has to have illustrations to really have educational value. Very few people are going to read the signs if they are just text and nothing visual or interactive. If they are just pictures, people are going to look at them, but not learn very much. It is a matter of getting a balance between the two that works.
Even the shape of the sign can be important. You laugh, but kids are more likely to look at signs that are of a natural or interesting shape. If it is just a rectangle, it won't attract as much attention even though it is more cost effective to produce. Similarly, large, colourful illustrations are important. Maps are also useful, especially when accompanied by a label saying the region in which they are found. Also essential for all endangered animals is saying they are endangered and informing the visitors as to why. For example, on a sign relating to orangutans or Sumatran tigers, it might be a good idea to have a section on palm oil, and then something that tells visitors which products might have palm oil in them. Not many people will read it, but perhaps a small percentage of those people will think more about palm oil before they buy something.
The facts that are added to the sign are also important. If you regurgitate a Wikipedia article onto the sign, no-one will be interested. If the information on the sign is varied and a bit niche, like a weird fact or strange adaptation, people will read it and remember it. And though they might not know the absolute essentials on all the animals, at least they will come away with some information on the animals at the end of the day.
Another thing that can be useful is having several different but smaller signs. That way, they potentially get a lot of information across but look less daunting. A lot of panda exhibits do this really well, with bamboo around the exhibit (it may look like useless theming but it engrains their diet) lots of signs and perhaps a size comparison. Of course, this all costs a lot of money, but not when you look at it in comparison to the enclosures themselves! It is really good for education at the zoo and reinforces yet another reason why zoos are needed.
. I think it is a Nile crocodile. In fact, a book on reptiles did exactly the same thing. Had a massive photo of a Nile croc on a double page about American alligators, pointing out the 'short, rounded snout' on the long, pointed snout of the croc 
