Also I've just asked this question in the JR/SE and Bobby/Pam threads but since Donna & Ray are the third great couple to ship from the show I figured I'd ask it here too:
For me I think it's that this couple have this opposites attract thing going for them. On the surface they are very different, she's a lady, he's a cowboy. But if you look a bit underneath the surface they are more compatible than one might think. They have both lost their parents at an early age, they both want the same things in life, they truly enjoy each other's company and they love each other. Another thing about this couple that is great is that they seem so down to earth and we often see them doing everyday things such as washing the dishes or riding the ranch and such.