Comfort. You will have to carry your messenger bag, loaded with a number of items, and how comfortable the straps and design is comes on top of the list.
Storage. Size and placement of the main, auxiliary and side storage pockets and spaces is very important. The bag should serve the purpose of containing what you expect to carry, and slightly more when you need it to.
Durability. Purchasing a bag each and every day is no fun, regardless of the price at each purchase. You need a bag you can rely on, without having to collect the contents from the pavement every Monday morning.
Versatility. The number of ways the bag can be used determines utility. Can you go camping straight from the office, and can you turn the laptop compartment to carry a couple of clothes for a sleepover? A bag which interacts seamlessly with other luggage is also considered versatile.
Security. Does the fastenings, zippers and Velcro claps provide sufficient grip to protect the contents from loss? Do you have to remove your laptop at the security desk?
Professional look. A professional look comes into play, considering you may be forced to do your elevator pitch in the elevator or street, or get the call from your office on your way to coffee.
Weather proof. Changes in weather may come unpredictably. How capable is your bag from protecting the contents from the weather when you are not able to? Does the accidental coffee spill cost you a laptop and a bag as well?
Ease of Cleaning. The ease with which the bag can be cleaned is one major criteria for an excellent bag. In this feature, dirt-proofiness also counts, since no one likes a bag which has to be cleaned every weekend.