What's wrong with the prosperity GOSPEL is that it substitutes worldly pursuits for spiritual pursuits as the "good news" (the meaning of the word gospel) of God, rather than the good news of spiritual redemption/salvation by the death of Christ. In other words, Christ didn't die so you can buy a nice house, a nice car, and send your kids to college. Society says these things are evidences of worldly "success". But what does this say about believers who are poor? That God does not love them as much as prosperous folks? It is too easy for people to judge holiness by who has the most possessions rather than serve God by serving those with the least possessions. However, Chrsitianity does not forbid worldly success. Hard work, thoughful investments, and material abundance is not sinful per se. It's just that the wealthy must remember those who don't have material abundance and to be fair-dealing, honest, and generous with the abundance.