As others have said, beware of just having one pack that will do it all. Instead, focus on the type of trips you'll be taking most often. When I got started in backpacking, I bought a monster, 6 pound pack that would carry two weeks worth of gear. Then I realized, after a few months, that nearly all of my trips were short, 3-day or less trips because of school and work demands, and that massive pack was just way too much overkill. I've spent the last couple of years adjusting my gear collection to reflect that. I now have a much smaller pack that's capable of handling the amount of gear I use for 80-90% of my trips.

Now, the school/work situation is changing, and I'll have the freedom to start taking some longer trips, possibly even some expeditions. My plan is to use the gear I have as much as I can, and rent the gear I still need. It's much cheaper to rent a monster expedition pack for the one trip you take every three years than to buy one outright, given their cost. If, however, you start making more and more of those expedition trips, it might become more feasible to buy one.

I guess what I'm trying to say is...think about not just what you want to do someday, but what you're going to do a lot of in the meantime. Good luck with your search!