I have many camera bags. From 3 Billingham's, 3 Think tanks and numerous backpacks. I just can't seem to find the one I Like. everyone has good and bad. But this is what I have learned so far.
1) I like the bag to be stylish. If I like the looks I will be happier with it and also grab it more often.
2) Protected from the elements. Rain or snow. I don't want to baby the bag or worry about it.
3) slightly larger than what I need. I like taking a 3 lens setup when walking around most of the time. 3 primes, 3 zooms, or a combination of. Room for accessories. I don't like things to tight that I have to struggle getting in and out of the bag. Or have to pack in a certain way in order to fit.
4) Confortable and not to heavy. The lighter the bag the easier to carry.
5) *Have a spot for a water bottle. This one has become a big must for me. I find when spending a day or half a day shooting I always end up getting a bootle of water and never having anywhere to put it. I don't want to hold it my hands as I want to shoot. Placing it in my bag during the summer forms condensation or can wet the inside. This one I find more and more of a must.
Just my two cents worth to consider when looking for a bag.