Critics (mostly in Europe, where ACTA has caused sometimes violent protests in the real world and online) claim that gives one country the right to reach into another to prosecute someone for behavior that may be legal in the second country.
European companies, for example, might be able to prosecute a U.S. web site for not handling either software or customer data according to the laws of that particular company.
European critics claim ACTA violates European Union laws against such intrusions and violates the privacy and data-protection rules for which Europe is famous. It may violate U.S. laws for the same reason.
(Here's an explication and discussion of ACTA with much of its language intact, on the site of the anti-ACTA, pro-digital-rights group Knowledge Ecology International.)
The anti-counterfeiting rules and decision to treat illegal downloads in the same way as illegally manufactured pharmaceuticals warps the treaty enough that any enforcement would almost certainly violate U.S. laws.
One major objection is that ACTA was negotiated in secret and signed, other critics charge, in violation of U.S. law, which gives the president no right to negotiate intellectual property treaties unilaterally.
According to the anti-ACTA Electronic Frontier Foundation (EFF), the effect of the treaty would be "to create a new standard of intellectual property enforcement above the current internationally-agreed standards in the TRIPs Agreement and increased international cooperation including sharing of information between signatory countries' law enforcement agencies."
Reddit's list mistakes devleloping Free Internet Act: (Sheesh, aren't you trying to make any money?)
It's easy to attribute too many virtues to people we agree with and negative intent in those we don't.