To upgrade, or not to upgrade? That is the question. And when it comes to upgrading, it all depends on what your interest in the hobby is. There are bashers, drifters, racers, crawlers, scale creationists, and even people who like to pimp their RC out. Depending on your preference and what you want to do with you RC ride, you're going to want to explore the options sooner or later. You might want to spruce the ride up a little or maybe you broke a part and now you are replacing it with an upgrade.
I consider myself an RC racer and occasional basher. When I make an upgrade, it is usually on one of my race models and it is in the hopes of higher performance and quicker lap times. When it comes to racing, one of the easiest and most important upgradable parts is the tires. A simple change from the stock tires to a respectable tire manufacturer, such as Team Sorex for asphalt or AKA and Proline on dirt), will result in excellent hook up on the track. When people think that they need to go faster, they often turn to changing the motor and that sometimes leaves them with too much power and not enough handling to keep the wheels on the ground.
When it comes to toughening up a vehicle, one of the best options is aluminum upgrades. Many RC manufacturers offer the most vulnerable parts as optional aluminum upgrades. Team Integy is another company, aside from the manufacturers, that offers aluminum parts for many different RC vehicles. If aluminum is not the way you want to go, you can always choose tougher plastic or composite parts, such as RPM RC Products or T-Bone Racing.
Basically, I look at it this way, you always have something to do with your RCs. You could be happy with your setup, but want it to look better - get a new body and paint it up, or put some rims on it that look nice (I recommend anything black because I was told that it'll add .5 secs onto my lap time just by being black - LOL). You could be happy with the looks and want to change the performance - upgrade the motor, turnbuckles, ESC, suspension arms, shock oil, shock springs, etc. There is always something waiting to be done to your RC.