Best features for me:
1. comfortable hip belt
2. doesn't hit your head when you look up or go uphill when you have it packed full.
3. comfortable on the countours of your back/lumbar area
4. accessible one-handed water bottle pockets (or camelback)
5. removable lid/waist pack/day pack?
6. good foam in the shoulderpads/hipbelt
So for some of those reasons, you should really try backpacks out in the store.
I have a Dana Design, but I think they were sold and Marmot manages them now.
I got my pack before the ultralight movement, so it's kind of heavy. I wish I had a lighter pack.. if you can shave off some weight before you even start packing it's good thing.
If you try on expensive packs, you will probably end up buying one. So either don't go there if you don't want to spend the money, or be willing to buy expensive if that's what's most comfortable. You will be wearing it a lot on a trip. Comfort is key.