Women's backpacks are specifically designed to accommodate subtle physiological differences such as shorter torsos and narrower shoulders. They are also built to fit more obvious differences between women and men: breasts and hips.
Many women need shoulder straps that are located closer together than on a man's pack. Straps that are too far apart tend to slip off the shoulders. Tightening the sternum strap may help, but it's uncomfortable when pulled too tightly.
The padding on standard shoulder harnesses is typically too wide, straight, and long for women. This may cause chafing in the armpits and restrict the chest. MEC sells some packs that have replaceable shoulder straps. Look for narrower, shorter straps with an angled yoke designed to conform to a woman's upper body.
Standard hipbelts don't always match the flare of a woman's hips. This may cause the bottom edge to dig in while the top of the belt sits awkwardly or simply doesn't rest on the hips at all. Loosening the hipbelt to make it more comfortable will transfer the pack's weight to the shoulders. To carry loads comfortably, the hipbelt should fit snugly over your hipbones and its curvature should eliminate spots that dig in. Models with interchangeable hipbelts will allow you to find the correct size.
Selecting the correct pack length is extremely important. If a pack is too long or too short, the hipbelt sits in the wrong place and the shoulder straps will carry the brunt of the load. Improper pack length can also make back and lumbar padding gap and bulge in the wrong places, adding to discomfort.
See Backpack Fitting for more information on good pack fit.
A few of the packs sold at MEC have unique features that allow them to be adjusted for a customized fit.