I'd say that while a single comment on a forum might not be enough to identify you, the sum of your comments can easily do (because of pieces of personal information you let slip in this or that comment); then everything you write must be considered personal data, unless there is a way to tell apart or remove the personal information from your comments.
Think about pictures: the picture of a landscape you took on a holiday doesn't identify you, but other pictures in your library might do (they contain faces, places, etc.). All together, the pictures can tell who you are and where you've been and when and with whom.
Some of these are very direct like your name and address. But since other personal data can be combined to create an identifying fingerprint of you it may or may not be covered.
There's a gray area in between of course were you really need to check the legal situation, but public content on a public forum is definitely not covered.