Seconding the waterproofing thing. Most backpacks leak through the zippers unless you start spending big money.

Also I've yet to meet any backpacks that are really durable as a proper "messenger bag" that cost less. That goes for stitching, materials, etc.

After carrying one for around 10 years now day in, day out, I feel naked without it. I'm pretty comfortable with any/all the short comings of the bag.