Like I said, I am not a lawyer. I'd be grateful for any detailed analysis of ACTA as compared to current German law. There's an insane amount of hyperbole on either side…
One basic problem with something like ACTA is that mostly it's "encouraging" countries to think and debate about "appropriate" measures and laws. A lot of the language is about "might" "should endeavor to" etc. – and it probably has to, or you might just clash with national legal concepts. So just signing it might imply a response from "Will do sir! Let's get the jackboots out!" to "Yeah, yeah, sure, we'll all have a nice chat about it…".
Still, signing it is a bad show of intent, a truly worrying symptom of current politics. It opens doors better left closed…