After a day of hiking in the rain, finding that your sleeping bag and packed clothes are also waterlogged is about as much fun as finding that your dog has confused your bed with his toilet. Weatherproofing your backpack is critical. Few people plan to go hiking and camping during downpours, but it's good to be prepared if one comes up.
Most backpacks have a water resistant interior coating, but water resistant does not mean water proof. Big difference. Plus, that coating won't do much good around the packs zippers and other openings. You can try waterproofing the exterior of your pack yourself for additional coverage, or you can get a rain cover for your pack. Rain covers are water proof and cover your entire pack, so water won't seep in zippers or other openings. Most backpack manufacturers sell rain covers that work with their packs, or you can go the cheap and easy route and slip a plastic garbage bag over your pack.
Carrying a backpack in the rain or snow can be a soggy experience, and so can carrying one on a hot summer day. The difference is, in the rain you get drenched with water. In the summer you get drenched with sweat -- and no one will want to share a tent with you, stinky. While your backpack is, like the name says, going to ride on your back, if you do a lot of summer hiking you may want to look for one that's a bit cooler. External frame packs, because they aren't molded to your body, can offer more ventilation between you and your pack, which can help keep you a little cooler.