I'm going to concur with TheJeepster on the USMC and Army packs. My brother bought a surplus ILBE pack and my wife has the MOLLE II pack she was issued in the Reserves, which I've worn in a couple of instances. Both have used these for week-long camping/hiking/canoeing trips where everything had to fit and still allow for good mobility. Now, none of us is 6'3" (my brother is 6' and I'm 5'11") but neither of these packs is going to be a problem for someone taller. The MOLLE II frame looked uncomfortable to me when I first saw it, but after wearing it while almost comically overloaded, it wasn't bad at all.
All that said, these are not my packs. I usually only need something for day hikes, which for quite some time now has been a Condor pack, although I do own an older Kelty Yukon external frame back (bought used from REI) and a "large" ALICE pack. While either one of those could carry a pretty serious load, the Army pack is far more comfortable and allows for a lot more flexibility. I've looked at the current selection at REI, and while there are some nice packs there, I think I'd opt for one of the military-issue packs for anything other than ultralight use.