Remember, you arent a surgeon. Your most common injuries will likely be puntcure wounds/cuts, blisters, and minor burn injuries. Possibly something like a sprain. Of course, if you have taken additional training, then pack what you know how to use. I have 3 kits:
EDC FAK, which is nothing more than alcohol wipes, assorted bandages, and some pain relievers, allergy med, and anti diahreal pills. Aqll contained in a mint tin

hiking/kayak FAK: one of the off the shelf ones from a good company, with gloves & a CPR mask thingy thrown in. This goes into my daypack, kayak, or whatever I am carrying. It covers almost everything I would encounter, and is light. I may even have an ACE bandage in there.

Jeep FAK: This one is pretty comprehensive, and has tons of bandaids, a couple ace bandages, a SAM splint, gloves, another CPR mouth thingy, and a couple bottles of painkillers, as well as anti nausea, anti diarrhea, and antacids in there. I also threw hard candy in there as well (Werthers), mostly because I like them, but I am SURE they will come in handy someday.

AMK makes great kits, and are usually well stocked. These are great to build off of. Thats my $0.02