Why doesn't Kuzco have a food taster? It's because he thinks the whole world adores him. The thought that someone might dislike him, let alone try to kill him, never even crossed his mind.
When Yzma's goons all get turned into a random assortment of animals, none of them seem nearly as alarmed as you'd expect... save one guy, who asks to be sent home early on the grounds that he is now a cow (Yzma allows it). Why is he the only one showing any concern? Well, despite what pop culture would have you believe, cows are exclusively female. Poor guy didn't just have his species changed, his biological sex was changed as well, which is disturbing and unexpected even by the standards of someone used to Yzma's transformation antics.
Yzma asks Kronk "Why do we even have that lever?" Well, really, what happens when Kuzco and Pacha try to sneak into the lab? Maybe that's why! Except that Kuzco and Pacha just get right past the crocodiles with little problem, so this happens:
Kuzco: Why do they even have that lever?
In a subtle bit of the opening scene, Kuzco looks back and slips- that's why he falls in the mud. It's an amusing metaphor for what he tries to do before undergoing Character Development.
Some Fridge Humour: Yzma's original plan probably would have worked so long as the potions weren't mixed up, but she decided not to go through with it because she wanted to save on postage. The entire movie only happened because she was too cheap to buy a couple of stamps.
As Yzma smashes Kuzco busts in a rage, she mentions that she practically raised him. Taking into account Kuzco's parents are probably dead if he's ruling, this could very well be true, and would certainly explain how he grew up to be such a spoiled brat.