Absolutely nothing at all! Single paned windows were the original window style, and provided you use a quality brand and installer, they represent a cost-effective, easy solution to your window needs. They’re also simpler to retrofit than double paned windows or triple paned windows, as they are lighter, cheaper, and easier to handle.
Why, then, do multiple pane windows exist? When you analyze the energy usage in a home, a lot of heat is lost through the expanse of glass in our windows and doors. Likewise, warm air from outside can enter. This means some of our energy costs are being ‘wasted’ on air that’s leaking to or from the outside. The more you can stop this, the more you can save on gas, heat, cooling, and other energy usage costs. Insulation is a hot new trend in promoting energy efficiency, after all. As more and more of us live in crowded cities, it’s also worth considering that the gas layer and two panes of glass used in double pane windows offers better noise insulating properties.
While these are important aspects, of course, they’re only of value to your project if they’re meaningful in your specific circumstances. Here in the Bay Area, we don’t have the extremes of temperatures that many other parts of the country do. While you may still want double pane windows for energy efficiency, it can take longer for the investment to ‘pay off’, as it were, than in a cold or very hot climate where the benefits to your bottom line are more immediate and noticeable. While many home- and business-owners are opting for double paned windows as they have to replace their windows, this is typically done to extend the lifetime of the installation, to fit in with the energy efficient ‘green’ trend, and, simply, because they can afford to do so. This does not mean that your single pane window will be an ‘inferior’ installation, simply that it does not offer some of the extended features of other window styles.