Okay, I'll try to generate a checklist, but please, everyone, feel free to tweak/expand it.
Good first aid knowledge and PRACTICE! - take a course that includes simulated, practical sessions.
aspirin (blood thinning for stroke or MI)
tylenol - pain relief
acetaminophen - pain relief through a different mechanism
Benadryl - for hives, congestions, hay fever, anaphylactic shock, and as a sleep aid.
Imodium - for the squirts (diarrhea)
tiny professional sample of triple-antibiotic cream.
some laxative - if you don't know a dozen natural laxatives you can find on the trail
Know how to use:
sleeping pads as splints.
clothing as dressings, slings, and traction devices
pack frame, branches, and hiking poles as splits, etc.
knife / scissors / razor blade to assist in preparing such field dressings.
Know how to identify and more importantly AVOID environmental exposures (hypothermia, frost bite, heat exhaustion, heat stroke) using the gear you normally carry.
Have appropriate water treatment and use it, if needed.
Hand sanitation scheme used before every meal/snack and after every elimination.
Sun lotion, hat, long-sleeve clothing.
bandaids - 2 to 4
moleskin, molefoam (and if someone has totally untested boots: second skin) and scissors to prep them (e.g. SAK Classic).
Duct tape in a few wraps around your water bottle. Good for sucking chest wounds, securing dressings, splints, etc.
Some people swear by super glue in lieu of stitches. The MD I sleep with says stitches on the trail are for the Rambo-wanna-bes. You shouldn't stitch something up unless it has been irrigated and cleansed in a way we just can't do well in the field. And waiting a few days has no downside once the bleeding has stopped.
Longer trips: anti-fungal cream for athlete's foot, jungle rot, ringworm, etc. Again, a tiny professional sample.
Medicine history / knowledge of the people on the trip. This is often overlooked, but is REALLY helpful with an unconscious victim.
Specific drugs based on the individuals along. No known bee sting, peanut allergies? I'm not bringing the sting kit. But if someone has a chronic or acute condition, I'll stock on the expected drugs as well as some for contingencies.
I would rarely bring ALL of these. But based on trip length, group size, remoteness, and individual issues, I'll consider each one carefully. On a 16-day, 16-person rafting trip of the GCNP (including an obviously gay dude who wouldn't disclose his health status), we brought much more, but the raft floats no matter how much you put in it and we had two MDs along.
I'd rather have a creative, knowledgable first aider with only the non-first-aid items on his/her back, than an untrained idiot with an ambulance full of equipment.