[Big Papa Goat]

The big unique feature of gnosticism is that it makes the god of salvation alien and the god of creation evil and sub-godly. This basically denies the transcendent origin of creation, which is one of the essential points of christian theology.
The gnostic process of salvation is also pretty different from the christian view in my opinion. The gnostics thought salvation was acheived by the recovery of mans alienated pneumatic divine spark. Basically, it's about getting your divine essence back together after its 'fall' into the body and psyche. I don't recall a lot of that kind of symbolism in the salvation process that Christians talk about. Christian transcendent salvation is obtained through the grace of god through the sacrifice of Jesus Christ. Jesus and his sacrifice redeems humanity, but does not recover anything for us. Knowledge of Christ, prayer to Christ, the act of Christ's sacrifice do not cause human beings to get their original essence back in order from the disorder caused by the material world or our own sinful nature. Our sinful nature doesn't change at all, and we remain perfectable only in death.
So where the gnostics thought human beings could recover their essence through acquaintance (gnosis) with an alien god (acheived through the mediation of jesus in the christian heretical view- jesus seems more like a prophet than a sacrificed savior in this view, his role is to communicate with men about the divine so that they can get an understanding and start on the road to recovery, rather than redemption)

So ya, basically my thought on the difference between salvation for christianity and gnosticism is that christianity is about redemption through the grace of god, while gnosticism is about recovery of the spirit through acquaintance with god.

Then there's the differences between their creation myths, but I'll have to post about that later.