Depending on how old your home is, there are any number of improvements that could be beneficial. Some of the more common are: Electrical, Windows, Insulation, Plumbing and Appliances. While the upgrades are not vital to living in your home, they could have the long term impact of reducing your monthly expenses while improving the quality of your home.
Electrical[]
If your home was built prior to the 60's, chances are good that all your electrical outlets are two-prong only. It wasn't until the 60's that home electrical systems were installed with a ground wire. This can be a problem with equipment that have three-prong plugs. Adapters do exist, but they aren't recommended for critical equipment. So an eventual upgrade of the electrical system could be a good idea. Also you might want to check your electrical service panel. If it is more than 20 years old, it is missing any number of safety improvements that have come up since it was installed.
Windows[]
In addition to letting in light from the outside, windows are the means of seeing the outside world from the comfort of your own home. Reasons for upgrading range from changing how your view is framed, to improving how well insulated you home is from outdoor weather and noise. Unfortunately windows are an important source of leakage and can add to the heating and cooling bills. If you have an older home, the odds are strong that you still have single-pane windows. Particularly where temperature swings can be sizable, replacing single-pane windows with dual-pane windows can be a good move.
Insulation[]
Does you home change temperature rapidly to match the outdoors? If windows aren't the problem, the the addition of insulation is probably what is needed. Start with whatever attic space you have, and then see if the external walls need insulation. In addition to controlling temperature swings, the right kind of insulation can also reduce the amount of noise being transmitted through walls.
Plumbing[]
This is your water transport system. It not only moves water to where you want it, it also keeps it contained. You want to be sure that it has adequate capacity to meet your needs. You also want to know that there are no leakages, as they can lead to dry rot problems.
Appliances[]
This generally refers to kitchen and laundry equipment. Main reason for upgrading is that the appliance is no longer operating as it should and repairs are no longer a viable option. Increasingly, however, age may be a reason. This is especially true of electrical appliances where newer models are often more cost-effective to use, than older models. This is because of significant technological improvements that weren't availabe 20 years ago, and in some cases, 10 years ago even.