The problem was that if the court declares Breivik insane, Norway couldn't (legally) put him in a high-security criminal prison, and there was no equivalent for psychiatric cases. Now there is.
It is a change, and clearly in response to this case, but the scope of the change is limited.
That's wrong - they can do that without Lex Breivik. The issue is when he's declared healthy, what would then happen? It's legal to detain him if it can be proved that he constitutes a danger to society. However, if he is not considered a danger to the society, he will be able to walk freely. With Lex Breivik, they will be allowed to detain him as long the society is a danger to the person.
Think about that for a moment. With Lex Breivik, you can be isolated even if you've not done anything illegal (or have finished serving your imprisonment) or is considered healthy, because some people in the society want to do you harm.
From an article on the opposition to the law[1] (my translation):
The Department of Health hasn't tried to hide that the
goal is a stricter set of laws which can be
used if Breivik is sentenced to compulsory mental
health care.
The department has long wanted a change of laws to
improve the security in institutions that hold highly
dangerous patients. This work was sped up after July 22.
The criticisms I've seen seem to center on that the new law makes it too simple to declare someone "dangerously insane".
Do you have a link supporting your interpretation?
[1] http://nrk.no/227/dag-for-dag/lovforslag-moter-kraftig-motbo...
[2] http://www.aftenposten.no/nyheter/iriks/Lex-Breivik-splitter...
Take a look at http://www.regjeringen.no/nb/dep/hod/dok/regpubl/prop/2011-2... - esp. § 4A-8, which states the new changes in the law.