Agency Agreements
Americans are often shocked by European agency law.  Hire a European agent for a fixed period and, even when its contract expires, the agent can be entitled to compensation!

Certainly your standard USA agency agreement will not have been drafted with European legislation in mind.  The laws relating to commercial agents in Europe have (more or less) been harmonised and the position of commercial agents in relation to their principals has been strengthened.  In particular, agents can benefit from mandatory minimum notice periods of up to 3 months and are entitled to compensation on termination of the agency agreement, even if the agreement is terminated on notice, or a fixed period expires.

This very much contradicts the USA approach of freedom of contract, as it is simply not possible to contract out of these mandatory provisions.
Intellectual Property Issues

American technology companies acquiring European companies can suddenly discover that the IP crown jewels, which they thought they were buying, belong to a third party.  In most European jurisdictions, unless the IT contractor assigns his/her intellectual property rights in his/her developed work to the software company, the IT contractor retains ownership of the intellectual property rights in the work.

As a licensor, you simply can not apply the kinds of all-encompassing prohibitions on reverse engineering, decompiling, etc., commonly used by US licensors.  European legislation permits limited access to licensed software to ensure interoperability with a licensee's existing system.  It is not possible to contract out of this right.

Consumer Protection
Whilst many major sales channels involve sales from business to business, many US corporations are now beginning to reach consumers.  There is a whole raft of legislation that is aimed at protecting consumers, some local and some Europe-wide.

The member states are at varying stages of implementing this legislation. But care must be taken, where channels deal with consumers, to ensure that the terms on which your goods are sold on your behalf by your partners are fully compliant with the latest changes in this area.

Data Protection
A variety of European laws now regulate the processing of personal data and data privacy.  Your sales channels will need to comply with the data protection laws in every country they operate in.  The USA is less robust in its attitude towards data protection, and so your contracts are unlikely to address these issues.

This list could go on, but enough: I think that the point has been made.  You use your USA contracts at your own peril!