If the spear had gone through his heart or spine, he would have died instantly. As it obviously didn't, I can see how he could have been saved through modern conventional medicine.
Absolutely right. The problem is that fiction simplifies the process of death, both for story convenience and to sanitize it somewhat, and thus people whose only experience with violent death is what they see in the media tend to have a lot of misconceptions about it. In fiction, people -- at least bad guys and extras -- tend to drop dead instantly after being shot or stabbed, but in reality, it's often a lot more gradual depending on where the injury occurs. People have survived being shot by hundreds of bullets or stabbed dozens of times. Some have even survived being impaled through the brain, though not without personality changes.
There's also the fact that medicine has improved, and thus has pushed back the limits of what constitutes irreversible death. Coulson was in the hands of the medical team moments after he appeared to die, and people in real life have been brought back from worse. And we only have Fury's word over the radio that the medics had "called it," i.e. declared him dead. And we know Fury lies.
That said, it wouldn't surprise me at all if Joss went with something a little more "out there" to explain it.
That has been implied in interviews, yes -- that the mystery of Coulson's revival is something that will be developed gradually over the season. So while it could
be something as simple as the doctors getting to him in time and being very good at their jobs, it probably won't be.