Long Tarmac Delays are a hot topic on the Hill and in the Halls of DOT. Legislation may in the works and the DOT has proposed a rule on the issue.
There are many reasons the plane does not return to the gate and allow passenger to get off the aircraft. The main one being that doing so, will actually create a longer delay if there is a sudden break in the weather. The aircraft is no longer in line to take off and then would need to re-board, taxi back out and then get back in line.
I am not saying that are right or wrong, just that there is a lot more variables than most passengers are aware of.
The New York Times just published an article on how to best cope with these events, and unfortuately, they will continue. Even with legislation in place.
http://www.nytimes.com/aponline/2009...lays.html?_r=1