I think you will find that other countries extradite to the US quite frequently based only on a basic outline of the case and often not even based on Prima Facie evidence. Then again I fully support Britain tightening some of it's extradition treaties and particularly the 2003 US/UK Treaty. Perhaps if the US does refuse a basic criminal extradition case, it may lead to a catalyst for change in relation to extradition between the US/UK, Italy/US and even EU Countries and the US and that would be something I would fully support.
In terms of the UK 2003 Extradition Treaty with the US, I would welcome the following -
1.) Someone should not be extradited to another country for actions that are not criminal in the UK.
2.) A basic case (Prima Facie) should be made to a British court before someone can be sent abroad to face trial in another country.
3.) If a significant part of the conduct that led to the alleged crime took place in the UK, then a British court should be able to decide if it is in the interests of justice to extradite.
Such changes to the extradition law may have prevented extradition to the US from Britain of individuals in recent years such as Giles Darby, David Bermingham, Gary Mulgrew, Christopher Tappin, Babar Ahmad, Talha Ahsan, David Carruthers, Ian Norris, Jeremy Crook and Peter Dicks. Whilst Gary McKinnon and Richard O'Dwyer extraditions to the US have thankfully been refused by Britain, and US extradition requests in relation to the extradition of Paul and Sandra Dunham and Dave Mcintyre are still pending. As for Australian national Julian Assange he remains hold up in the Ecuadorian Embassy in London with a 24 hour police guard payed for by the British taxpayer. Most of thes cases above being cases largely involving actions committed outside of the US, usually on British soil by British nationals or involved actions that aren't deemed criminal in the UK.
In terms of murder cases that have taken place in the US, there is usually no problem with extradition. Although if the US wants to start politicising murder cases, I am sure the Italian and other countries will be more than happy to reciprocate. It also should be noted that the US extradites more people than any other country in the world and if any one benefits from the current arrangements it's the US rather than countries such as Italy or the UK.
Extradition Watch | Liberty - protecting civil liberties, promoting human rights
Amanda Knox Extradition Should Be Granted | New Republic