Why is there so much controversy about stem cell research?

Stem cells are essentially "blank" cells that haven't grown into something specific yet (like a nerve cell, skin cell, etc). You can get them from adult bone marrow or from fetuses. There is REALLY promising research in this field in terms of curing degenerative and fatal diseases. My question: why are some people so against this research if it means curing illnesses? Yes, fetuses are living beings. But if they've already been aborted (as in some cases) why is it still considered wrong to use the cells for medical research?