1. What skill level is the user? Basic skills = basic first aid tools and supplies. EMTs, Nurses and Doctors feel more comfortable with different and/or more advanced equipment in their Trauma kit.
2. What is the environment? Your home has different hazards and resources than say sailing alone across the ocean. Therefore you will need to respond to different first aid issues and the supplies need to reflect that.
3. How many people do I need the kit to take care of? A family of 4?  25 people on a church retreat? A business accident with 200 employees, etc. Kits that say 25 person DON"T mean that they take care of 25 injuries or even 1 Badly injured person.
4. What first aid supplies do I need ?   Caring for say, a broken bone with a quick arrival time may require very little, but with serous bleeding or burns it may use up a small kit in minutes. A cheap kit that's from a grocery store does not have supplies for more than small cuts and scrapes.
5. What first aid supplies do I need until advanced medical help arrives in a disaster? How long will it take for an Ambulance to come to your location? In a normal city may be 5 minutes. You might drive to an ER and be seen in 20 minutes. But during a disaster (an earthquake, flood, snow storm, hazardous materials, etc.) with hundreds of calls it can be many hours for an ambulance and very long day at a hospital parking lot whith hundreds of others to get even minor care.  If an ambulance can't even reach your neighborhood, do you and your neighbors have what they need to care for each other?  Does the designated disaster shelter in your area have actual emergency supplies including first aid on-site?  Or did you expect Red Cross to bring this days later? 

Everyday move than a thousand people are displaced from their homes and businesses by disaster and emergencies. Don't wait to think about these things. Get training, get ready, get supplied, get piece of mind!