If patients cannot be denied coverage for pre-existing conditions what keeps a patient from purchasing health insurance after they are diagnosed with a condition requiring expensive treatment?

The primary problem with Obamacare that many people miss is that its goal was to provide universal access to healthcare, rather than universal healthcare. In other words, it was supposed to eliminate many of the barriers that were preventing people who wanted or needed it from getting it (pre-existing conditions, being dropped if they became sick, establishing exchanges to make it more affordable, etc.). However, at least for individuals, the tax penalty for not carrying insurance begins at $95 and (I believe) increases annually.  http://www.businessinsider.com/how-much-is-the-obamacare-penalty-tax-2012-7  The problem with this is that for many, especially younger, healthy people, it will still be more cost-effective to pay the penalty than paying for insurance plus their usually low medical bills out of pocket. Universal coverage is the only way to ensure costs are spread among all degrees of healthy and unhealthy people. The problem with universal access is that people can (and will) opt out, and the penalties are insufficient to see coverage as the better option.

This is likely to happen and prompted Ezekial Emmanual's editorial in the Wall Street Journal pleading to healthy young Americans to shoulder their responsibility. http://online.wsj.com/article/SB10001424127887324326504578467560106322692.html The mandate penalty was decreased to minimize backlash during the vote despite arguments that if the mandate was cheaper than purchasing insurance, young healthy Americans would pay the penalty and opt out. This will only serve to increase the cost of insurance to those who need it. "Then there is the benevolent Obamacare regulation called “guaranteed issue.”  That requires all insurers to cover everyone who applies, no matter how sick they are when they first apply, without having ever paid any premiums to the insurer before.  That concept applied to fire insurance would require fire insurers to cover applicants who waited until their home caught on fire to call for coverage." "That regulatory requirement is then paired under Obamacare with the further compassionate regulation called “community rating,” which requires insurers to charge all applicants the same price, no matter how sick when they first apply, except for sharply restricted variances for age, geographic location, and smoking.  Applied to fire insurance, that concept would require the insurer to charge no more for the applicant that calls with his house already on fire than for other applicants." http://www.forbes.com/sites/peterferrara/2013/04/07/look-out-below-the-obamacare-chaos-is-coming/

Nothing prevents it. That is the point, deeply buried inside the mounds of compromises to an attempt at healthcare reform. The prevailing attitude in the United States , especially on the right, is that healthcare is primarily an economic issue. While it certainly has many implications for the economy, I feel like that conversation and attitude in our country is that healthcare is a privelage.  When people fail to see how others having health care coverage benefits everyone, the direction of policy goes askew. I wish Obama had just held out for single payor. I think having healthcare insurance tied to employment is courting disaster. Check out the Frontline special on "Obamas Plan." It just drives home how powerful the insurance lobby is in our country. Let me give an example of how being myopic about healthcare can affect a community adversely. My husband and I both worked in inner city Detroit in a hospital that treated a variety of people, including the indigent. We saw the patients that the suburban hospitals stabilized and transferred out. This hospital was the end of the line, so to speak.  My husband is a pharmacist who now works in epidemiology and informatics.  About 15 years ago, MRSA was a hospital acquired infection.  Many of the OP clinics and community resources had been shuttered by the state.  So, patients with MRSA often did not follow through on treatment. MRSA became a community acquired infection. MRSA is not pretty. My husband is a Canadian living in the U.S. He really loves a lot of things here, but there are times when he comes home from work and tells me he finds the policies and attitude about healthcare here as inhumane and shortsighted.

To discourage this- you cannot apply outside of the Special Enrollment Period. However- there are enough exceptions to become qualified under a Special Enrollment period. If a young person gets into an accident outside of the special enrollment- he or she cannot buy insurance. A young woman who gets pregnant- can qualify for her new baby- but not for herself. If a person stops working due to Cancer of  an Accident- he has had a Life Change Event- due to drop in income, and losing coverage. However- you are required to continue your COBRA coverage. Not paying your COBRA coverage for even one month- automatically disqualifies you. So the must buy COBRA rule can be dodged. But there is a transition period in all of these loopholes.  So count on a $20,000 bill if you get rear ended. Or shoot yourself while cleaning the gun. Or if Dick Cheney shoots you accidentally. You can always "move"  to another state- and buy an insurance plan that has coverage in your "previous" home state.  Remember that your move could be ruled fraudulent. You will also be cheating the American people- but then you may  also have breast cancer, and getting half your breast  removed. So yes- there are loopholes. But they are not very good loopholes.