To some extent, OkCupid is just joining the modern era. Numerous other dating apps require your first name, so you're not left wondering how to address someone when you meet them in person. And of course, creeps may be more willing to message you if they can hide behind an alias.
However, the decision reduces your chances to maintain your anonymity. Real first names theoretically make it easier for harassers to track people down or attack them based on their gender, ethnicity or sexual orientation. Facebook's initial approach to real names created serious problems for women, the LGBT community and others who are trying to avoid harassers, stalkers and bigots.
OkCupid tells Engadget that it takes privacy "very seriously" and notes that it has taken some steps to make it difficult to creep on others, such as removing profiles from its Google search index. It also reiterates that it's not asking for anything more than your first name, and that you can use a nickname if you prefer. You still have some wiggle room to remain elusive. Even so, the policy shift gives you one less place where you can completely mask who you are until you're comfortable revealing more.