GDPR is sooo easy to follow as a startup. Just gather the data you need and not everything else,and ask for consent.Clearly we're not going to ask for consent to track someone who is systematically probing our site for vulnerabilities, or someone who is attempting to use us to validate presumably stolen credit card details, or a group who are obviously sharing a password to gain unauthorised access in violation of our terms of service.
Also, the purpose(s) of data processing matter, not just the data itself. It's not as simple as only gathering what you need. You also have to ensure that what you gather is used appropriately, and that you have the means to respond to the various rights that subjects have by law.
Thousands of people on mailing lists with no control of how they got there.
Actually, that was one of the tricky areas when the GDPR came in, and something almost no-one got right despite good intentions. Specifically, the widely accepted best practice for managing a mailing list had long been to use double opt-in, thus verifying that the subscriber really did intend to receive the messages, and to provide a simple, automated unsubscribe facility. However, unless you had kept all the confirmation replies, under the GDPR you might not have met the required standard for evidence of each list subscriber actively opting in to receive your mails.
That led to a wave of messages being sent out to mailing lists asking subscribers to confirm they still wanted to receive the mails. This was particularly ironic because if those subscribers hadn't already intended to consent then those messages were probably themselves in violation of existing law in much of the EU even before the GDPR came in. The difference was that before, no-one was seriously worried that a legitimately operated mailing list with double opt-in was going to be targetted for business-crippling penalties, but with all the ambiguity around the GDPR and the uncertainty around how it was going to be enforced, a lot of people panicked.