A diaper bag is many things: a place to store diapers, a burp cloth, and a change of clothing for baby, and also a handbag for mom (or dad). As baby gets bigger, it also needs to store snacks and sippy cups.
So there isn't anything magical about a diaper bag that makes it different than any other bag in your closet. Some have special features, like being easy to wipe clean, or having the right sized pockets, or having many pockets, but what is right for one mom may not be right for another. Likewise for the same mom- one day one bag may be right, but another day you may need something else entirely.
My ideal diaper bag has a zipper pocket for my wallet. I also like a bag with a couple of compartments. I like to keep the diapers and wet bag separate from my water bottle and baby's extra pants, just so I can get them quickly. The extra carrier can go anywhere. Sometimes I like a bag with a separate place for toddler (and, ahem, mommy) snacks, and soemetimes I put these into an insulated lunch bag in the main compartment. I also carry a blanket to use for keeping baby warm or covering up when nursing, and keep this in the main area. Sometimes I carry a camera, which goes into a neoprene case in the main area to protect it from falls and moisture.
When I am out with the baby and preschooler, I have been using a messenger bag or plain backpack as a diaper bag. The backpack's advantage is that I can wear it on both shoulders. Sounds silly, but with a baby in arms (or in a carrier), and a toddler who needs hugs or to be carried sometimes, it is often the diaper bag that makes me feel like I have too much to carry around. The backpack helps alleviate this feeling.
I like to travel light, and always have. Sometimes I underpack, but I like to think I pack just what I need.
When I am just out with the baby, I carry my purse. In it, all I need is my wallet, phone, keys, baby timer (I am a space case without it, sadly), wet bag, and 4 diapers. The carrier goes separately and blanket and extra clothing get left behind.
Interestingly enough, I could also go out with the same handbag with my toddler, though it would only last for a short trip because his food takes up so much space. If I would do this, I would bring my wallet, keys, phone, extra pants for him, an apple, and a packet of almonds.
For both kids, I need my wallet, keys, phone, baby timer, carrier, blanket, extra pants for both, diaper cover, 6 prefolds, wet bag, water bottle, 2 toddler snacks (2 fruits, almonds or walnuts, yogurt or beef jerky or smoked salmon or roasted chicken) and a mommy snack (piece of fruit and some nuts). I also carry children's hats for the cold or the sun, depending on the weather. If we used disposable diapers instead of cloth, I would also need wipes (I use the prefolds) and burp cloths (I use the prefolds), and possibly plastic baggies for soiled diapers. Some moms carry pacifiers as well, and others include hand sanitizer, tissues, or a mini first aid kit. I share a water bottle with my toddler, but some kids prefer their own, so some mommies carry multiple water bottles. Others carry toys for their kids (I only do this for doctor visits and airplane trips). I already mentioned the occasional camera we tote around. Some moms also carry a changing pad, but we do mid-air changes, since we have so few poopy diapers.
What's in your bag? What makes the bag itself special? Do you have any non-negotiable diaper bag features (like my zipper pocket)?
p.s. here are more of my diaper bag posts (they are all links): petite diaper tote, hobo bag, detour diaper bag, convertible tote, 241 tote