Long story short they tried to fit a full season of an English show (six episodes worth of plot) into a full length American season, which required them to have several writers all working on different bits of the season and lots of irellevant padding. This led to them constantly adjusting the rules of how the immortality thing was even supposed to work, flip-flopping on what the moral quandry was meant to be and just generally mishandling the whole central dillema.
Examples: Near the beginning Jack says the immortality epidemic thing can't be anything to do with his blood which was stolen decdes earlier because his immortality has nothing to do with his physical makeup or blood, then the entire thing turns out to hinge on Jack's blood after all. Near the beginning everyone who doesn't die due to the "miracle" is apparently still aware and conscious despite extreme physical damage (a guy who gets torn to shreds by a bomb is meant to still be aware) but then later people who should have died of heart attacks or other old-age maladies are shown just lying unconcious on hospital beds. There were plenty more but the show aired years ago so I can't call them to mind.
