Let's take an overview:
The medical kit depends on many factors--in order of importance:
What is your knowledge level?
How far are you from a rescue?
How big is your group?
How long are you gone?
What are the common problems you are likely to encounter?
The aim of medicine is to prevent problems first. So encourage your trip members to prepare ahead of time (fitness), bring the specific medications they need, and to take care of small problems (blisters, thirst, cold) before they become big problems (cellulitis, dehydration, hypothermia). A medical kit should be designed to treat the treatable problems you're likely to encounter. If someone falls and has major head trauma or internal bleeding they is probably nothing you can do for them but get help.
First GET KNOWLEDGE: It's the most important thing in your kit and weighs nothing. You'll be able to do more with a 6oz first aid kit after taking a wilderness medicine course than you could do with a 2# kit without training.
Then tailor your kit to your trip: On most trips (2-3 days, evacuation 1-2 days away) blisters and minor injuries (cuts/sprains) are by far the most likely small problems that can turn into big problems so a variety of tapes and bandages are first on my list. Also benzoin to help it stick. Rarer is more major trauma and generally splints can be made from branches and clothing. Pain medications (non-Rx and Rx) can also make a big difference in comfort and ability to travel under own power. Allergic reactions can be treated with Benedryl (non-Rx) or an oral steroid--if a trip membeer has had anaphylaxis before they need to carry their own epi-pen. Altitude illness is somewhat less common and most types can be prevented or treated farily easily with Diamox or descent of 500-1000'. Infections are less common, but I carry 1-2 types of antibiotics as it can make a big difference in comfort, the ability to travel under one's own power and survival. However most infections are viral and won't respond anyhow. I throw in a few safety pins (allow you to use your shirt for a sling, keeps an open airway), a pair of gloves, syringe and needle (draining pus, relieving a tension pneumothroax and know how to do an emergency trachiostomy with a swiss army knife (however I do have 4 years of medical school under my belt). I'll look through my kit when I get home tonight and add anything else I find.
The important message is: Get Knowlege first and you can lighten your kit a lot through technique and improvisation.
After reviewing my kit I'd like to add a few more comments. First for "wound care" I bring Steri-Strips which hold a clean/sliced deep wound together well--I saved a trip to the ER after slamming the bridge of my nose on a metal support upstairs in Ryan's gear loft (bathroom downstairs, lights out, trying to navigate in the dark), a tape called Leukopore which really sticks well (duct tape just doesn't) an assortment of bandages including circles (great for fingertips with a slice down the side to seal better). I also bring a topical antibiotic--they sell small foil packages which can be used multiple times. I also have a couple of compeed patches for blisters.
I carry Doxycycline (and if on a longer trip Ampicillin also) for an antibiotic, Benedryl, Vicodin, Aleve (long lasting with 12 hour dosing=fewer pills), Loperamide (anti-diarrhea) Decadron and diamox (mostly high altitude problems but decadron works for allergic reactions that last a while). I also have a small flashlight and some micropure tablets (for water and wound cleaning ). I also carry a few windproof matches and some heavy string and fishing line with needles stuck in a small piece of insulite pad. Finally, a small glue stick (not really first aid but very helpful if you step on your glasses--it will burn and drip hot glue where ever you need it.