Never has so much bollocks been talked about a treaty
Myth #1: It has not been ratified by the US
It was ratified by both sides in 2007 http://www.usembassy.org.uk/ukpapress48.html
Myth #2: It is easier for US prosecutors to meet the standard of proof required for an extradition to the US, than for a UK prosecutor to get someone extradited to the UK.
Not true. The US prosecutor is requried to show 'information which would justify the issue of a warrant for the arrest of a person' and the UK guy has to show "probable cause". These are a similar legal standard.
Myth #3: Gary McKinnon would not have been extradited under the old arrangements.
Not true. Under the old arrangements the US would have had to show prima facie evidence. His admission of guilt goes way way WAY beyond that requirement.
Myth #4: He cannot challenge the evidence.
His team have tried various challenges including this, but the evidence is easily enough to meet the standard for extradition. The proper forum for challenging any evidence now is in court (in the US). The fact that he's doing his best to avoid this tells a lot.
Myth #5: He was in the UK when his actions are alleged to have taken place, so should be tried in the UK.
If his actions are an offense under US law then he can be tried in the US. This is no different from if you were standing in Dover and shot someone in Calais. You would be tried in France.