Re: The Fine Line Between Dry and Busy Sounding.
When it comes to pads, I EQ and duck (sidechain) the hell out of them, and drench them in reverb and a nice thick phaser. In my stuff, the pad is pretty much non-existent when there's a lead, bass, drums, and maybe a rhythmic stab playing, but you can just hear it if you listen for it. When it really shines is during the breakdowns, as it carries the chord progression without any other instruments. I often write my songs as basic chord progressions on a pad before adding other instruments, so for me it's more an issue of making everything else fit the pad.
A note on sidechain, I have the kick do straight volume modulation of the pad, but most other instruments do frequency-dependent volume modulation using multi-band compression, other wise there would just be no pad during the full sections of the song.
The other way I know of to fill out a dry mix is to dial in some delays on a few instruments, maybe a layer or two of reverb (usually on an instrument by instrument basis) to thicken up the midrange. With delays, set it by ear, with reverb (on my setup, which is headphones), I set it by starting at the bottom and coming up until I can just hear it for the first layer, then taking it back. Turn it off and on to confirm it's doing something, then you can add a big fat hall or a plate if you want.
I set saturation/distortion on many instruments throughout a mix much the same way, and keep that part towards the beginning of my FX chain so that I can still EQ out any rumble the distortion might add or accentuate if it were after the filter.