I don't think there can be a single answer to obesity - there are many reasons people become overweight, and why they have trouble losing weight. But I think some pretty simple changes could really impact obesity rates.
- Making nutritional information clearly visible on the menus of chain restaurants. It won't stop anyone from ordering Wendys, but it might make a difference in WHAT and HOW MUCH is ordered if a person is confronted with the calorie count BEFORE food is eaten.
- Make access to gyms and fitness centers available to everyone. A lot of rural areas don't even have gyms - not even YMCAs (I know my parents' town doesn't). Moreover, even where they are available, they're often expensive. My gym membership is currently $25/month, and that's SUPER cheap, but at my heaviest I could have never even afforded that little. Statistics consistently link obesity and poverty, and I think the three key elements in that are cost of food, cost of fitness, and culture.
- Make healthy foods more affordable. This is slowly happening as more and more people demand organic/natural foods. However, it's still hard for people on tight budgets to eat healthy. The cheapest products are also heavily processed. My parents really struggle to keep my dad's diabetes at bay while keeping in budget.
There are certainly a lot of other things that could be done to help. But I don't think that taxing junk food is going to do much good at all - having a negative with no positive alternative isn't going to encourage people to get healthier. We need to make a healthy lifestyle the norm of our culture, and make it available for everyone.
An afterthought: I really think the FDA could do a better job of managing or even stopping the diet supplement industry. Every time another crap "supplement" is advertised on TV my stomach turns. These supplements are presented as legitimate, as having some kind of medical basis - even going so far as to having "doctors" appear in their ads. They feed on the desperation of people who feel like they have no control over their weight, and justify the misconception that there's an "easy" way to get fit. Combined with the junk food industry, it's a double-exploitation of an entire population.