I don't think ACTA is harmless. Here are my concerns:
1. Threat to British Innovation: investors will be reluctant to invest in UK Internet start-ups (this talk is on SOPA, but I believe the same applies to ACTA:
http://bigthink.com/ideas/42035).
2. Harmful to the British Economy: in these times of austerity, I believe ACTA will make every day things much more expensive because behind the scenes companies will be using the mechanisms within ACTA against their competitors. One example that immediately springs to mind is the cost of running the NHS. I don't think it's a far stretch to imagine many legal 'generic drugs' being re-classified as 'counterfeit drugs' under ACTA. This would result in a massive increase in the cost of running the NHS.
3. Wave of US style litigation: Any country legally bound to ACTA will have to promptly divulge the personal details of any "alleged IP rights infringer" at the request of a rights-holder. I believe if ACTA passes, we will see a wave of US companies suing UK/EU citizens. We will not, however, see UK/EU rights-holders suing alleged US infringers as ACTA will not be legally binding in the US (the US will not ratify it as it violates existing laws/constitutional safeguards). It's another non-reciprocal treaty that was formed and promoted by US companies, and that favours US companies.
4. The UK/EU will lose the ability to shape and form its own IP laws. What works in the US doesn't necessarily work in the UK/EU. Regardless, if ACTA is ratified, the "ACTA Committee" will dictate future UK/EU intellectual property laws.
I believe the mechanisms within ACTA could even be used against the Raspberry Pi project (after all, Raspberry Pi uses Linux, and Microsoft claims Linux violates a number of its patents).
I don't think it's 'harmless' at all, and I for one would like my government to conduct a thorough impact analysis before ratifying this treaty ("impact analysis" - not just an ECJ opinion).