Paganism is much better than nothing, as we are beginning to recognize, but requires completion by Christianity.
From a Christian perspective, I venture to suggest three main things wrong with paganism: pride, cruelty and meaninglessness.
Pride is the worst of sins for Christians, it is to prefer oneself to God. But for pagans, pride is a virtue - it is, indeed, the only thing holding life together and providing some kind of motivation.
Cruelty is not a virtue for paganism, but it is an endemic feature because there is no prohibition of cruelty - therefore when cruelty is expedient or pleasurable there is no compelling reason to refrain.
(Christians are also cruel, obviously - but this is against the core imperative of Love: so by being cruel they are not being Christian; whereas cruelty is compatible with paganism.)
Paganism offers no meaning for life - or more exactly no human meaning for human life - or the human soul.
Paganism offers only a spark of life in an infinite universe of darkness, or the 'hope' of being extinguished or absorbed into nature, or recycled - at any rate there is no human meaning, no preservation of the essential self.
The pagan soul may be eternal but is not individual.
Pagan immortality is merely an indefinite extension of human life, perhaps more focused on pleasurable things.
(The Christian hope for salvation and eternal life is of the human life made God-like - individuality preserved but enhanced, eternity made not merely bearable but wonderful. Indeed, I am convinced by Pascal that properly-considered *only* Christianity offers a meaningful life - and that no other religion or philosophy does so.)