Mostly loops and lack of variety, most of us are pretty heavy in the pattern-recognition thing. If a creature only has one "hit sound" it's going to sound really repetitive and lame. Two or three variations, even if they vary ever so slightly, is a lot better. Same for ambient sounds, and, well, everything. Even a bird that only sings one song is going to sound slightly different every time.
I just realised that you might be able to combine several sounds into a cicada pattern (like they do visually here http://www.sitepoint.com/the-cicada-principle-and-why-it-matters-to-web-designers/) to avoid obvious looping.