The biggest single thing you can do to affect your general level of health is not smoke. THe second is pay attention to your diet and exercise.
BMI and weight are guidelines, not facts. Some professional athletes, for example, can fall into the slightly overweight category because they've bulked up as a result of all their training. Likewise, very slim people will have a low hip to waist ratio, yet this is usually an indication of a weight problem. There are always going to be people who don't fit the rules, but as a generalisation they hold true.
It is perfectly possible for a person to be up to 2 stone overweight with no affect on their health as long as they are eating well and exercising regularly. In fact, a little excess weight helps the immune system and is a valuable defence against illness.
Beyond that 2 stone or so, however, the chances are that health IS going to be affected. Most people who claim otherwise are in denial.
I know quite a few people who are overweight who regularly exercise and eat a good diet, but it is obvious (to me, at least) that where they're going wrong is the sheer amount of what they're eating. Portion sizes count for a lot, and they've got bigger as a result of food being more available and cheaper than at any time in history, and the fact that we tend to measure them against portion sizes as provided by manufacturers (all of whom are making their portions bigger to encourage more spending - family sized bags of this and that, and extra large ready meals, etc).
I also know quite a few people who eat excellent healthy meals, but counteract it by eating between meals. Apparently one of the main reasons diets fail is because people consistently underestimate the calorific content of their snacks, even though they may go as far as weighing the ingredients of their main meals.