So far there have still been very few articles in the MSM describing exactly how the new healthcare system would work and what exactly it does.  From what I'm able to piece together your cancer patient example would be handled the following way:

Cancer pateint A buys insurance from company B.  They cannot be denied coverage and cannot be charged extra for their pre-existing condition.  Company B's aggregate risk goes up and thus their cost go up. 

This cost increase is dealt with in 2 ways.  First the mandate within the bill increases the number of people in the insurance pool, spreading the risk over more people paying premiums.  The second way is there is a mechanism for risk adjustment (I don't think this is the right term, but I forget what it is).  Basically it works sort of like the luxury tax in baseball.  All insurance companies pay into a risk pool and those funds are redistributed based upon the risk profile of the different individual companies.  Thus companies that have mostly low risk patients pay more into the system than they get back.  Companies with more high risk individuals will get more money back than they pay in.  It is supposed to be a way to remove or at least decrease the incentive for insurance companies to deny care to unhealthy individuals.  

The fact that Dave doesn't understand why mandates are in the bill infuriates me. 

We want universal health insurance coverage in the United States. 

To get there we need to reform the insurance industry. 

Let's outlaw pre-existing conditions and rescission. 

Fine, but if you do that wouldn't people just wait until they are sick to buy insurance? 

Yes, so there must be an individual mandate. 

Is that fair to require people to purchase insurance?  What if they can't afford it?

We will provide subsidies and expand already existing programs to help people out.

Wow, that's going to cost  a lot of money.

We will pay for these reforms completely by reducing costs in Medicare and raising new taxes.  In fact this plan will save us $1.3 trillion dollars

And America becomes a country with universal health insurance coverage, and we get more coverage for less money?

Yup.

Let's do it.

Wow, how to begin on this. This country has ways of progressing to things you wouldn't expect, at a pace which is sluggish, but there also have been many things in my 46 yr old lifetime that have went backwards at the same time. I never thought I would see a time where stupidity is worn like a badge. 

How to know how this will shell out... One thing I will say, if progressive salesmanship doesn't improve drastically, and if we do not unite as the teabaggers did to demand what we voted for, my guess this will be the "moving backward" aspects of our lives and culture I mentioned earlier.                                                                                                                                   Damn Democrats.... we had all the tools we needed, except the spine.

"Life's rich demand creates supply in the hands of the power, the only vote that matters." --James Michael Stipe, from Begin the Begin.