Well, first off I want to talk about Glenn, and Carl getting his eye shot out
Glenn is on-top of a wall with Nicholas, and Nicholas shoots himself in the head while falling on Glenn. The ending scene appears to be Walkers tearing Glenn apart, but that's not the case. According to some YouTube videos, Glenn is underneath Nicholas, and the Walkers are tearing Nicholas apart. The reason Glenn is not being harmed (my theory) is because in the second episode of the first season, they cover each other in the dead's intestines so that they are allowed to walk through the city safely. So, being covered with Nicholas' guts/insides may have allowed him to slip away since Nicholas was already dead.
In the comics, Carl was supposed to get his eye shot out during a raid. Douglas Monroe, is said to be shooting, and either hits a Walker and it goes straight through it and hits Carl, or the bullet just strays; either way, the series is going to have some pretty pissed fans, and fangirls. And if it did happen, Carl will probably murder Douglas, and that wouldn't be good.
[/color]This is my theory to why Carl, Rick, and Dayrl haven't died-[/font]
[/color]Most likely, they are very valuable characters and the most 'main' and 'important' characters on the show. Also, once again, fangirls are an issue. Because, I KNOW that if Carl died, a ton of people would be very pissed off. (I wouldn't really care if Rick died) Dayrl dies, people would be really pissed off as well as Norman Reedus. And, nobody wants a pissed Norman Reedus.