I tend to think of Ps as "wanderers" and Js as "directed." I agree with Lurker... although I only know one INTJ and he is very driven, he also knows his goals specifically (like getting into a top-notch grad school) and goes for them (by doing a lot of sports/other activities and seeking out internships and stuff over summers). I also know only one INTP, lol me, and I have a much more circular and open ended learning/working style. I basically want to do what I love, and I love to find out about the world around me. That leads me to go "hey... what about the physical world?" and study physics. Then "hey... how do people see the world around them?" and find a bunch of different people and spend nights (almost everyone is more open to discussion after 1 or 2 in the morning) talking to them. Then "hey, does science really cover all of reality?" and look into logic theory and theology. Most recently it's been "how do feelings affect people and is there any 'right' or 'natural' way to experience them?"
Ps can be very hard workers and pour tremendous amounts of energy into things--I've actually skipped food for a day without realizing it because I was too into something at the time--but we're much less decisive about what it is we want to/should do to accomplish the goal. We're also really easily sidetracked (I have a P friend who would come and play Bridge with us at 12:30 even if he had a 4 or 5 page paper due in 13 or 14 hours that he hadn't started yet), but the smarter ones seem to rarely fail to get something done "in time" (although our personal "in time" may be different than a deadline a teacher would set, but it's usually a conscious choice to not worry about meeting their deadlines so much).
Then there're a few other normal things you can read everywhere... Js are more organized and tend to find/make some order in their lives... Ps will drop stuff wherever there's room and deal with it later (ie deal with it never). INTPs at least will drop things anywhere that will be "out of the way" and ignore it until it's needed or gets in the way of something else. To Js it looks horribly disorganized--and it is--but we (again, the smart ones at least) always know where everything is, as long as we were the ones who dropped it there. If someone goes in and moves something or organizes it for us, it's maddening because now we don't know where the stuff is anymore--even if they explain the order to us.
Ps are at our best when we're allowed to wander and work on things when we want, as opposed to any fixed schedule. If I make something a sort of life goal, say, "learn about the world around me," I'll make progress that leaves other people stunned, because I can just mix work and play naturally. If you stick me in an office or classroom with stuff I don't care about, though, and say "ok for the first 15 minutes of class we'll be doing X, then get together in groups and discuss Y..." telling me exactly what's needed, I'll tune it out almost every time and dismiss it as irrelevant--after all, I'll find out what we're doing when the time comes, right? Why worry about it now? Js, on the other hand, don't seem to understand that, and would rather know what they're getting themselves into beforehand, and feel more comfortable when there's a plan set in place. Basically, Ps are more "circular" in their learning and "figure it out when the time comes" people. Js are more "focused," "directed," and "plan ahead" people.
at least... that's what I've been able to gather. I only know 3 Js, and they're all TJs... so my generalizations might be influenced more by seeing the thinkers, so it may not include you guys... but to the best of my knowledge those are the differences.
oh that's right, I forgot... I think Ps tend to have lower self-esteem too. That's not because it's a P trait, but just because the world doesn't seem to value the P style of learning very much anymore. That may just be because I have two strongly J parents, however, who thought that people who didn't plan ahead and know exactly what they were getting into are foolish. I think it's the same kind of reason that most INs have low self-confidence too, because introverts are (or at least I was) raised to think we're supposed to want a lot of friends, and intuitives are classically misunderstood. The combination of few people understanding your thoughts and not sharing them much in the first place tends to make you doubt your mind a lot. I think Ps are under the same kind of effect... not that there are few Ps, but just that P-ness isn't valued as highly.
