[1] http://en.wikipedia.org/wiki/Extradition_Act_2003#Controvers...
The treaty was controversial in part because it did lower the evidentiary standard required to extradite to the US (in other words, it did make it easier to extradite to the US). What the controversy didn't recognize was that the former treaty had been unbalanced.
A legitimate concern though is about forum and jurisdiction. The treaty currently allows the US to request extradition for acts committed in the UK that are not a crime in the UK, but there is no corresponding entitlement for the UK to make such request to the US. Many people in the UK feel this is an encroachment on UK sovereignty, and not something that the US would itself offer to another country.
(‡ shame it had to be rounded down, rather than up)
Clearly Roman Polanski also believed Sweden less likely to extradite him than the UK, since he has actually visited the former (where they did arrest him, but then released him without charge back to France).