What's your hiking style? Your preferred hiking destinations, style and season will determine your choice of footwear.
How sensitive are your feet? The more sensitive your feet are, the stiffer a sole you'll need to protect them from rugged terrain. This goes double if you're backpacking; you need the stiffness to protect your feet under the extra weight.
Do you need a waterproof membrane? If you often hike in wet or cold conditions, a breathable waterproof membrane will keep water out and release sweat as it accumulates. However, when water gets in it can't get out and the boot can take a long time to dry when emptied. It also tends to be hot in warm weather, so look for this feature only if conditions warrant.
Shop later in the day. Your feet tend to swell throughout the day and on long hikes, so shop once you've already been on your feet for a while; otherwise, your "just right" boots may turn out to be too small.
Err in favor of a larger size if you plan to hike long distances. Your feet may swell a half-size to a full size larger than usual during these trips. This doesn't mean you should size up from a perfect fit, but if you're in between sizes, going up is usually the best choice.
Wear the socks you intend to wear while hiking. They can make an enormous difference in fit, especially since wool hiking socks tend to be thick. If you don't already have socks to hike in, purchase them when you try on your boots.
Test hiking boots while wearing a loaded backpack. This is especially important if you carry a heavy pack. It's the only way to be sure the boots will remain comfortable and supportive under your typical load.