What a brilliant question!

Well, the difficult thing about this is that everyone has their own idea of what a properly maintained animal, of almost any species, is. Take the indoor/outdoor cat thing or the dog in the bed/not on your life....oh,no that's just me isn't it! (LOL-see, used it again!)
My problem is that I am never happy that I'm doing enough - I'm alright with the dog and the cat as I know that they are telling me that things are hunky dory by their behaviour. The birds and fish have ever expanding areas to live in, although the fish (a marbled Oscar) has his space and stays there no matter how big the tank is.
Then, when you have made the space, you feel that the living space is too empty and a companion is needed. (For the birds, not the fish as he will eat anything that he can fit in his mouth and batter to death anything he can't!) Then, of course, you need more space......
All in all I think that reptiles and amphibians tend to be bought for the wrong reasons and then left to suffer in bad housing and with terrible diets.
Birds of prey are the next for exactly the same reasons.