The problem was that Dark Souls wasn't a game where you figured out how much damage you could deal while your enemy dealt damage, it was about learning enemies' movements and learning how to react. So although increasing enemy damage and health in NG+ made it so that it was possible to die, it really didn't give a different, more difficult experience.
Much more thought should go into NG+, by not only increasing enemy health, but also changing where enemies are in areas and adding more enemies and ambushes in general.
Boss fights shouldn't just be lengthened by more health, but should have active changes, perhaps by adding smaller enemies to spawn gradually, like undead warriors for the taurus demons, or changing something else, like making more gargoyles spawn for the gargoyle fight.
NG+ infinity isn't something you should lose track of, since you hardly notice 7% increase, this changing of the enemies and placement of them in the world should change with every +, and always making it harder.
For instance, an Anor londo esque area should start finding itself saturated with silver knights, and gargoyles should the player go far enough.
This stupid thread gives an idea of what I mean