Access speed is the most underrated bag feature
A bag that requires unzipping three compartments to reach your camera means you'll miss shots, plain and simple. Side-access or top-flip openings that let you grab the camera one-handed, without setting the bag down, consistently get used more than backpacks that bury the camera under other gear.
Weatherproofing is about the zippers, not just the fabric
Marketing copy often emphasizes a bag's water-resistant fabric while saying nothing about the zippers, which are usually the first point of water entry in real rain. A genuinely weather-resistant bag has covered or water-resistant zippers, not just a coated exterior fabric.
Packing for airline carry-on limits
Camera backpacks marketed as 'travel' sized don't automatically comply with every airline's personal-item or carry-on dimensions, and international budget carriers in particular enforce stricter limits than US domestic carriers. Check the bag's actual exterior dimensions against your specific airline before you fly, rather than trusting a 'fits most overhead bins' claim.
What's worth buying beyond the bag itself
Extra batteries are the single most common thing new photographers underbuy — mirrorless bodies in particular tend to have shorter battery life than photographers expect coming from a DSLR. A basic lens-cleaning kit and a couple of spare memory cards round out the accessories that get used on nearly every outing, well ahead of items like elaborate tripod systems that see occasional use at best.