From the article linked to by the OP:
Much of the criticism seems to stem from perceived vagaries in the original legislation, which states that anyone working "frequently or intensively" with children or vulnerable adults should have to register with the ISA.
They aren't 'perceived' vagaries at all; the legislation is widely drawn:
"Definition of regulated activity: Any activity of a specified nature that involves contact with children or vulnerable adults frequently, intensively and/or overnight. (Such activities include teaching, training, care, supervision, advice, treatment and transportation.)" 
I.e. pretty much anything that involves children or vulnerable adults.
"'Regulated activity’ is when the activity is frequent (once a month or more), ‘intensive’ (takes place on three or more days in a 30-day period) or overnight." 
Concerns have also been expressed over whether the "balance of probabilities" civil burden of proof, used by the ISA to assess whether someone poses a risk, could lead to people being barred based on unsubstantiated allegations.
Um... the ISA isn't a court, they won't be concerned about the civil standard of proof. Jebus. The standard of proof referred to here is intended as an analogy, not as a legal mechanism by which someone is properly assessed and, if appropriate, convicted. And no-one will have their day in court in which to provide their side of the story to a neutral and informed decision-maker - I have no doubt that the authorities will err on the side of caution for the sake of the children.
I'm sure there was some over-egging the pudding, but the law is
widely drawn; it seems a bit much to expect parents, carers, famous authors etc to hope
that aren't required to submit to the scheme.
 http://www.crb.homeoffice.gov.uk/faqs/v ... cheme.aspx