Matt,
yes, its always going to be about differences as well, specifically, differences in density resulting from differences in volume arising from differences in temperature.
As for the chin-ups, your statements make sense although I see them as clouding the basic issue- thermals cant rise by themselves. My own expanding girth makes chin-ups a gravity defying issue as well.
Perhaps if I tried to tell you hot air rises mainly "because" of sink, would you re-consider it an important issue?

Tuno,
it is displacing the balloon because it is denser and therefor heavier, thus affected more by gravity.

toad,

Light is fine. Lighter is better still.
Ingo Renner once told me a typical thermal weighed some 50 tonnes. For the discussion, 50 tonnes or 1 gramm act the same, they are both affected by gravity and on their own, go down like a new bride.
Bouyancy is a better way to describe the way a thermal rises and many have pointed to the links available. Others still dont get it although glider pilots have a higher rate so far! Hot air does not rise.

Eric,
Thanks for advancing us! I had used simple bubble thermals, presumably on flat ground for the easy example.
I really struggle with Waynes Rule and smoke columns, especially like the column from a simple cigarette or incense stick.
My best explanation is that the smoke is expanding in a column along its vertical length, being restrained for a short while from sideways movement by the surrounding "sink" but hey, I do struggle.
Waynes Rule must still apply somehow.
The dustdevils/willywillys/tornado/waterspout funnel also is not as simple as the bubble, I surmise however they all come from a kind of vertical convergience, or vertical wave type of action.
As the airmasses collide, they have nowhere else to go but up. Perhaps there is some kinetic energy in the rising mass to accurately start the swirling motion, I dont know.

Dscopel,
your ignorance is pardoned, I do not live in a land where there are courtesy clerks - nor do a great many on this list. Personal attacks aside, the yahoo group statement specifically apologises to the great scientists (archmedes, thanks, boyle, thanks, newton, good on you all) and the purists, I am not one.
I just see a fault in the way most think of thermal dynamics and want to get myself on the right page or everyone else to be on mine.
I reckon the great scientists would applaud me trying to educate either the world or myself.

Bagger