For up to a week I use a Granite Gear Vapor Trail. Generally, essential features for me are a built-in sleeve for a water bladder, and ample straps for synching-down, compressing and lashing my load.
No doubt you already know this and can see in this thread, what is "essential" really varies among users. Perhaps the only quality in a good pack that is truly essential -- and easily overlooked in the age of internet shopping -- is FIT. One pack's medium size will be different from another, and even if we both have 19-inch backs (length of spine from the "7th vertebra to iliac crest"), that doesn't guarantee that what I find comfortable and snug will feel the same for you. If you haven't done so, you should try out as many packs as possible in at least one good store, a few if possible. REI or similar stores will let you load up different packs with stuff and walk around the store with each to see what works for you. Even if you decide to purchase online, a good pack, like shoes, is gear that really needs to be tried out in-person before buying.
Internal frame is probably also essential for any serious backpacking, 2+ days. Beyond that, there are two main types of packs: those with lots of pockets besides the main compartment, and those without. The former is preferred by those who like to keep things separately organized, and who need/want to access things fairly often along the trail or in the bush. The latter is for those (like me) who prefer to drop everything into one big compartment and don't need easy access throughout the day, and just unload for the night. These packs also tend to center the pack weight more closely inline with your spine (better balance), although this is not always the case. I like the GG Vapor Trail because it's basically an old-school rucksack design in new, ultralight materials.
I agree with Ming65, waterproofness and durability aren't significant issues. Most modern packs are made of water-resistant synthetics that easily repel even sustained light showers. Heavy rain (not safe to hike in, but it can happen if you need to reach point B before nightfall) can work its way through any exposed seam and nearly all zippers, however advertised to the contrary. The best protection is a rain cover that covers all zippers and seams, with relevant items drypacked inside. Of course condensation can happen in any pack, the more waterproof (non-breathable) perhaps the more likely due to poorly packed wet items (e.g., sweaty clothes). In terms of durability, even "ultralight" packs from the more reputable makers will easily survive ordinary scrapes and scuffs through trees and brush. Often, packs get damaged from the inside out due to poor packing. Sure, seams can come loose and material can tear on even the most bombproof pack, no matter how heavy-duty; but that's why you packed needle-and-thread and duct tape in your repair kit, right?
Hope this helps. Cheers --
Glen
