3 years ago I also decided not to come again, yet I did it this time; was a mistake.
It is overcrowded, because it's free. Queues in front of the crappy foodtrucks, coffee stands, rooms, everything, are there, because it is overcrowded. The feeling of dirtiness comes from the unpleasant buildings with tiny corridors, quickly applied concealing cardboards, ducktaped signs, underplanned amount and positioning of trash cans and the lack of enough cleaning staff.
We could explain for hours what other problems are stemming from the fact it is a free conference. However...
The organizers' unprofessionalism regarding crowd control, speaker introduction, moderation, maintaining the "OPEN/FULL" sign does not depend on money. I would think it's common sense, but apparently the moderators do not really care if in every 30 seconds there is someone sneaking in the room through a squeaking door. Every. Freaking. 30. Seconds.
Talks were great. All of those I managed to fit in the room, were fun, enjoyable, with a knowledgeable presenter. I felt extra bad for them trying to answer questions at the end of the sessions when people started to switch rooms because of tight schedules on scattered rooms.
Now I have an accountability buddy who would prevent me going to FOSDEM ever again, unless they introduce a fee of 50 bucks or something.