Quality manufacturer, painted all six sides, including behind the hinges with a quality primer and top coat and you should have no issues. As klem states been used for years as door and window material.

My concern with pine is that it's not a hardwood. But I really don't know much about doors.

I was going to get it pre-primed. Is American Building Supply a quality manufacturer? Both places here in town were sourcing from them.

Poplar, while a "hardwood", eg from a deciduous tree, is physically softer (eg more dentable) than many pine species. If you are looking for something that will be as resistant as possible to dents, oak is probably the most commonly available inexpensive door wood choice that is quite hard.

As the others have said, pine is a very common and serviceable choice for wooden doors. I would probably choose it over poplar.

Okay, great! Pine it is. How do I tell if I'm dealing with a quality manufacturer?

"How do I tell if I'm dealing with a quality manufacturer?"

I understand your desire to do this job up right but you are overloading on fine points. Afterall we are only talking about paint grade doors. By in large,the best value come's from local door plants. Reason being completed doors are expensive to ship. Vist a couple of shops that supply your local lumber yards and builders. I could reccomend the plant near me that I like but the freight would kill you. BTW,some species of pine are considered and graded as hardwood. Long leaf pine is a durable floor material secound to non in the price range.

First-time homeowner, just don't want to drop $3K on doors and then regret it. I guess pine will be durable.

Here is a chart showing the hardness(Janka scale) of woods common to the US(in yellow). As it shows, two varieties of white pine are the only pines softer than poplar.

The hardness is not the only scale for the usefulness of a wood. Cypress, for instance, is very moisture resistant.

http://tinytimbers.com/janka.htm