http://conovers.org/ftp/Everyday-Emergency-Kit.pdfProbably the best list for anyone without any experience (or even those with experience) to use as a starting reference. Just remember, training is as important as the equipment. I've seen plenty of doctors struggle to put on a tourniquet effectively. Buy and extra just for practice.
For the vast majority of people thinking about carrying an IFAK, it's like trying to use a full size glock as an EDC concealed carry. At first, it sounds like a great idea to have 17 rounds in a comfortable, full size gun. Then you start carrying, and for the first few days it's ok, but after a little while it just becomes annoyingly bulky. And eventually you stop carrying it.
A truck IFAK is another story. A tourniquet or two, some quick clot gauze (don't get the sponge), some regular gauze/gauze roll (kerlix), an Ace wrap or two, some gloves, a sharpie, +/- a pressure dressing, +/- a nasopharyngel airway. Should be plenty to hold you over in most basic trauma until EMS arrives. But improvising and knowing how to, and be willing to, stick a finger in a bleeding hole might do more than anything else you can carry in a bag.