rp, adding to what nick says, the main difference between an overnighter and a 7 day trip is that you cannot make assumptions about the weather, nor can you use the popular 'if my gear is inadequate return to trailhead and drive off in your warm car' plan B option, since you are too far from the trailhead to return to your car.
You need to think a bit more about food per day, make sure you know your fuel consumption per meal so you can bring enough fuel. Your body will also achieve a conditioning that will require probably more food than you are used to eating, for example, where I might eat half a cup of dry oatmeal with nothing in it at home, on a longer trip I will eat a full cup of chopped dried apples and oatmeal, same for dinner differences in quantity.
All longer trips where there is greater than 0 chance of rain you should have raingear that will work, in summer my feeling is that for relatively dry climates the dryducks top is light and good enough, though not robust, but very light.
You might also bring moleskins in case of blisters in odd places you never imagined a shoe could create a blister, the thick moleskin works better than the thin.
to me an overnighter is, check weather report, pack accordingly, that can mean 4 pounds less gear for me. You can also use a tiny pack on an overnighter since you are only carrying 2 meals plus maybe a lunch or two.
More water carrying ability too, depends, some people here seem to hike in areas where there are creeks every few miles, but in the heat, in summer, you want the ability to carry a few liters at least.
Also, you probably will in most cases want to bring bear bagging cord, for varmints or bears, that's something I think I'd leave at home on most overnighters. Depends on the terrain / ecosystems you will be in of course.
DEET is also something I'd bring on a longer trip, particularly in summer months, you can be in one ecosystem that is void of bugs, and 15 minutes later, be being bitten alive in another. Hat too, for sun, goretex socks for rain, basic stuff like that you might leave at home on an overnighter depending on the season and the ecosystem/climate type you are going to.
For me I don't usually think of overnighters, usually a 3 day trip is going to be my light load, and a greater than 3 day trip more stuff to handle more situations. Depends on the climate/ecosystems you enter however, and your experience with those.
Oh, I forgot the most important one, longer trips require greater mindfulness, every step, every action, because there is really no easy solution to a major error, a misstep, not securing your food and losing it, spilling your fuel, etc. That means you are paying attention, which means you leave the mp3 devices at home, hearing the sound of the ground you are walking on is a key indicator of trail stability etc, and also hearing a polite warning from a rattlesnake letting you know he is present before it's too late, etc. Being present and mindful at every active moment to me is one of the greatest pleasures of long backpacking trips, and every error/slip of attention is one that causes me generally major problems, either with nagging injuries or whatever else. Heat makes this type of slip much more likely by the way.