I’m there right now and it’s a wonderful event. Other conventions are commercial, in your face events but Fosdem is a welcoming, open event. Over 600 talks and some 8000+ people present.
I mostly attend open source/tech conferences in the Americas. I think I know what you mean, but that's not the main difference I see between FOSDEM and those conferences.
There's still "commercial" at FOSDEM (plenty of sales pitches in sessions, to begin with) but I think what's strikingly different is that FOSDEM has no "agenda", that is, the event isn't crafted around a theme and said theme doesn't dictate the rest of the content and the discussion.