There are a few things I look at to if a bag is good / great or just fine or even terrible.

All of them have to do with flexibility.

If you have used a Tom Bihn bag, you have most likely seen the o-rings they have at various places in their bags. I think they are kind of the perfect example of this.

They make it possible to have a key chain at any place where one of these rings are. And you can pick what kind of keychain you prefer. Plus if you break the chain it’s easy to replace.

Other bags I have have just sown some thin rope or other fabric into the bag at a fixed place. Some of them actually work well with this flaw, mostly because they have good taste for the length and placement of it. While others have ones that are more or less useless.