What special powers do they have?
The Nameks have many many powers that no other being has (with an exception of Cell). For example, Piccolo has the ability to make his body grow to the size of a small apartment building at will, and he can also stretch his arms out over 3 times its normal length. Some Nameks can do things others can't. But there is one thing Nameks can do, that no other being in the entire universe could ever even hope to do, that is create the Dragonballs. This basicly is the backbone of the entire series, from the very first episode, they have been after the Dragonballs, but for some reason, people see the Nameks as inferior. This may get off the topic, but lets imagine somthing, lets say if the Nameks never exsisted in the series. Then the DB series would not have had a meaning, and would pretty much have been a big flop, so then DBZ would not have been made, and DBGT would have never even been a thought. Then this site wouldn't be here, and then none of you would be making other sites, and pretty much the whole world would be different. So, what do you have to say about the Nameks now?
Why is a Namek the guardian on Earth?
Well, basicly, like I said above, when Kami and Piccolo Daimao slit, Kami was pure good, with not a trace of evil left in his body. With the power of good, and with the power to create the Dragonballs, he competed with the former guardians of the Earth, to see who would be the next Guardian. I do not know who all competed, but I know one person was Kami, and another one was Garlic(Garlic Jr's father). Garlic was evil, so he lost quickly. Kami won easiyl, so he was proclaimed the Guardian of the Earth. Then when Kami fused with Piccolo, Goku went and got Dende to be the next guardian. Because everyone knows that no one can beat a Namek as guardians, and everyone feels safer with a Namek up there.