I seem to remember we've discussed this very thing a number of times. Try the search function and see what pops out, then come back if that doesn't really answer your question.
But, to answer in brief; we carry for what we can reasonably treat in the wild. In my group I have 2 nurses, an EMT (Paramedic) and myself, a lowly Wilderness First Aid certified Joe.
We carry (as a group)
"horse tape" (disposable tensor bandages)
triangle bandages
EMT scissors
antibiotic ointment
betadine wipes
sterile pads
a small collapsible barrier device (for providing CPR)
a SamSplint (for broken bones)
Small amounts of over the counter meds; ibuprofen, anti-diarrheal, etc
Pack all into a bright red first aid bag.
As individuals, we bring a small amount of duct tape for blisters, whatever meds we might like and perhaps a couple of bandages for small cuts.
The rest is pretty much improvised on the spot. Tweezers, scalpel? See the guy with the Leatherman. Large fractures? Improvise a splint and do your best. Broken neck? Immobilize and get help.
To be honest, 99% of your injuries are going to be blisters (Duct Tape!), cuts and scrapes, twisted ankles and the like. You might get a broken bone, but you aren't going to be able to treat things too much more serious in the wild.
Injury and illness is no small matter when in remote places; a twisted ankle or bad water can be hellish if you're 2 days out and have no vehicles. I have a recurring IT Band injury that, if I'm careless when hiking, can cause me terrible pain. One trip, we found ourselves out in a steady rain with temperatures hovering around 5C. We pushed on to the next camp, arriving mostly hypothermic. The next day was good, but my knee was bad. The first 3 km of the 10 km to get out was fine, as it was all up hill. The 400m descent and 7 km remaining was brutal, even though we taped my knee and I did my best to keep my weight on my hiking poles rather than that leg. If I had a broken leg, we would be stuck and it would be a rescue situation, no doubt about it.