What You’ll Find in My Hiking First Aid Kit
Pain Reliever – Ibuprofen or whatever is available at home
Diarrhea Medicine
Moleskins
Gauze and adhesive tape

Here are a few other things I carry that can be used in first aid emergencies but have other primary uses:

Duct tape – can be used in place of moleskin, and can fix almost anything.  Some hikers wrap it around their water bottle.  This is a good idea, but will make washing the bottle more difficult
Needles – Sewing kit
Eye Drops – Like a shower in the morning
Hand sanitizer
Baby wipes – Clean all those hard to reach places and step out of your tent feeling fresh and clean
Bodyglide – This anti-chafe balm has removed a lot of pain from hiking

Comprehensive List
If you don’t mind the extra weight, or just want to be prepared for anything, add the items below to the list above.

Alcohol wipes
Antihistamine
Antiseptic ointment
Various bandages

If you have any other medical condition(s) you’ll need to prepare for those in advance.  If you have allergies to plants or  bug bites, diabetes, or anything else make sure your hiking partner knows about these issues in advance and knows what treatments to use that may save your life.
If you wear glasses you should consider an extra pair.

What’s in your first aid kit?