I agree that adverse selection is a concern and that the Exchanges won't work if healthy people don't sign up. That's why it matters how the program is designed.
You brought up younger people and I cited a plan that would be cheap and comparable in cost to the tax penalty. Of course, the high-deductible would be costly if the healthy person had a serious accident or illness. For the reasons you said, it would make more sense for a young/healthy person to pay, say, $250/mo instead of $150/mo and get an insurance plan with a manageable deductible and reasonable co-insurance (e.g., "bronze/silver" coverage).
I think most people who fit your criteria (are over the age of 26, are reasonably healthy, make too much money to qualify for subsidies, but somehow are not covered by their employers) who looked at their options would choose the latter deal. The per-month cost of the tax penalty would be $100-150/mo, and I just don't think most people like paying good money in exchange for nothing at all.
So I disagree that insurance companies will lose out on the healthiest (i.e., most profitable) customers. The individual mandate ("shared responsibility requirement") was designed to prevent this from happening. People have the choice to pay $150/mo out of spite and get nothing, or pay $250/mo and get decent health insurance. Hmm...
Penalty: The annual penalty for not having minimum essential coverage will be the greater of a flat dollar
amount per individual or a percentage of the individuals taxable income. For any dependent
under the age 18, the penalty is one half of the individual amount.
 The flat dollar amount per individual is $95 in 2014; $325 in 2015 and $695 in 2016.
After 2016, the flat dollar amount is indexed to inflation. The flat dollar penalty is
capped at 300% of the flat dollar amount. For example:
o A family of three (two parents and one child under 18) would have a flat dollar
penalty of $1737 in 2016;
o A family of four (two parents and two children over 18) would have a flat dollar
penalty of $2,085 in 2016 because the 300 % cap would apply.
 The percentage of taxable income is an amount equal to a percentage of a households
income (as defined by the Act) that is in excess of the tax filing threshold (phased in at
1% in 2014; 2% in 2015; 2.5% in 2016). For example:
o If an individual has a household income of $50,000, the percentage would be 1%
of the difference between $50,000 and the tax threshold (which is $9,350 for an
individual in 2010). Assuming the tax threshold is $10,000 in 2014, this individual
would be subject to a percentage penalty of $400. Because this percentage
penalty is greater than the flat dollar penalty for 2014 (which is $95), he would
pay the percentage penalty.
These penalties are pennies on the dollar compared to premiums. If I make $50k/year that means my penalty is $33/month. Then I can just buy insurance when I need it (and drop it when I'm done). Specifically, this is going to push middle and lower income brackets (those who are supposed to be the main beneficiary of this law) out of the pool because they have less disposable income.
People are always going to look at what the lower cost is. If my father could have paid $3000/year for insurance for him, my mother, myself and my four siblings, we would have gone on more vacations! Even if it was $5000/year (using the 2.5% rule assuming 200k/year) he would have been thrilled.