What a question!
How can you answer that?
It got me thinking - as a dub producer - perhaps I should be able to answer it, but I don't think I could talk for everyone.
Here though are some of the key ingredients I feel -
Deep smooth sounding Bass with punchy (if sometimes simple) melodies. Agreed that it's ok to mess around with the bass eq like Scientist in places - as long as it soon returns to the real dub bass sound.
As for dropping in and out and echo - well it wouldn't be dub without that. What makes that good - well echo timing is key. Delay (repeating) echoes can sound bad if they counter the timing of the song too much - Wackies tend to have the echoes trailing away in time with the beat - but it doesn't have to be like this - as long as the echo doesn't destroy the main riddim.
Dropping in an out is all about timing - I can't describe how this works, you just know when it is right - this kind of thing comes from feel. reggae music relies much more on 'feel' than any other music in my opinion.
MightyZ
I have tried to put my knowledge into my own dub album Mighty Dub - see how well I did:
www.last.fm/music/mightyz