The main thing to remember with creating a planetary cloud system is that the default scales really need enlarging considerably for each cloud layer.
Try this out in the density fractal; Feature scale=250,000, Largest scale=1e+007, smallest scale=2500.
That should give you nice big patches of cloud all round a default planet. If there's not enough detail in the clouds, use this fractal only as the 'blend shader' to a smaller fractal. This will make the smaller scaled shapes only appear where the large fractal exists.
This really quick image shows the settings above, without any secondary fractals. You could play with 'coverage adjust(colour offset if you use power fractals)', noise variation(which sharpens cloud transition the lower the setting is, and softens their transitions the higher it is set) warp amount, etc. to get a nice looking coverage. It's a start, at least.
* Don't forget to hit 'random seed' until you get lucky with the shape/distribution without needing to sculpt and plan the noise shapes manually or to use maps.