Any first aid kit I've looked at contains a lot of items I'd never use, and lacks a number of items I do use (for example, the bandaids in most kits are usually the lowest quality and don't stick). The most important part of any first aid kit is what's between your ears! If there's any way you can take a Wilderness First Aid course, I strongly recommend you do so. Check the schedule for the NOLS Wilderness Medicine Institute, check with the American Red Cross in your area (some areas offer WFA classes on demand); google for Wilderness First Aid classes in your area (colleges and universities often offer them). The standard First Aid classes from the Red Cross are not the same; they assume that paramedics are just around the corner (i.e. call 911). WFA classes assume that rescue may take several days and focus on stabilizing the patient until SAR arrives. Most WFA classes teach you to improvise with what's available, so you actually end up taking less first aid gear.

As mentioned before, there's no point in taking stuff you don't know how to use. It's just a danger to you and to anyone you might encounter.

Also, consider the statistical chances of various injuries. The favorite scenario of WFA classes, encountering a multiple disaster along the trail (such as a river rafting accident with lots of multiple injuries) is a very useful "final exam" for a class, but in real life the chances of encountering such a thing are probably less than 0.01% (and a good thing, too!). My own "injuries" have been limited to small cuts on my hands (usually the dreaded hangnail, easily infected), small splinters and the occasional blister (nonexistent since I switched from boots to trail runners and started using Body Glide on vulnerable parts of my feet). I have several times had to help beginning hikers with blisters. I did sprain an ankle once (X-C ski trip). My father was kicked in the arm by a horse when I was 9, leaving a nice hole in the flesh (from the horseshoe caulk) but fortunately not breaking the bone. I've had a few skinned elbows and knees, and of course my grandkids are more prone to that sort of thing. I once had an encounter with a yellowjacket nest (they nest on the ground), but fortunately my horse was stung, not me (he tossed me a long way from the nest, fortunately). There's always a chance of a yellowjacket encounter in late summer/early fall. My only encounter with a serious injury was when I ripped most of the ligaments in one knee during an X-C ski trip--fortunately with a large group at a cabin, where there was an evacuation sled. That's in 70 years of backcountry experience!

BTW, the WFA class I took 2 years ago showed us how to use a sleeping pad to stabilize a knee injury so the patient can hobble out. I'm glad I didn't have to try it, though!

My first aid kit:
-Bandaids, various sizes, most of the water block type
-Second Skin for blisters (also great for burns and abraded-type wounds)
-Moleskin for blisters (cut a hole for the blister and pad the area around it)
-Antibiotic ointment
-Tweezers
-Needle for sliver removal, with hole large enough to use dental floss as thread (in my repair kit)
The rest rarely get used but I don't like being without them:
-2 nonstick 2' x 2" gauze pads
-Tegaderm (non-stick) large dressing (1)
-3 butterfly bandages (for gaping wound)
-Duct tape (wrapped around one of my trekking poles)(adhesive tape substitute)
-Self-adhering foam wrap ("vet wrap")--originally bought for my dog, but I discovered that with duct tape reinforcement on the outside it makes a great sprained ankle bandage, much lighter than Ace bandage
-Acetaminophen (1 day's maximum dose)
-Ibuprofen (1 day's maximum dose)
-2 Rimadyl tabs for my dog, one day's dose (don't give pets either acetaminophen or ibuprofen; they're poisonous)
-2-3 Imodium tablets (enough for me and dog for 1 day)
-2 baby aspirin (to chew in case of heart attack)
-A few Benadryl capsules (enough for me and dog for 1 day) (for wasp nest encounters)
-2 Tums
-Two sanitary pads--the best compress (per the WFA instructors) for heavy bleeding (a case in which fast action is needed and you probably don't want to use your sleeping bag!)
-A few individually wrapped alcohol pads (for cleaning skin around wounds or disinfection before removing slivers)
-2 Ziplock sandwich bags--glove substitute and (with small hole in one corner) for irrigating wounds--never use iodine, hydrogen peroxide, alcohol or any other disinfectant in an open wound--it damages flesh and retards healing. Irrigating with treated water is the medically approved treatment. The latest from the US Army in Afghanistan is that pressure irrigation is "out," just flood the wound with copious amounts of clean water. Don't put antibiotic ointment down into a large wound, either, just a tiny bit on a gauze pad over the top. It also slows healing.
-Katadyn Micropur tablets, a day's worth in case of filter breakdown.
-Safety pins, large, 3 (in my repair kit)--if a sling is needed, pin sleeve to front of shirt.

That's what I take for solo (with dog) trips. For larger groups, each should carry a few items for themselves (especially meds) and one person should have a group kit with some extra items. Obviously, the more people in the party, the higher risk of a problem, especially if the additional persons are inexperienced or children.  

Since most of my trips are solo or with my grandchildren, I do take a PLB.

It's vitally important to check the kit every year, replace expired meds (just replace them yearly so you don't have to worry about dates), bandaids whose paper wrappers are yellow with age, etc. If you use the foam vet wrap, it should be replaced yearly--with heat and pressure it molds together into a single lump. The elastic in Ace bandage also loses its oomph after a year or two or if used.