Oh, a lot, actually.
The PCB production quality could be not good enough.
There are over 1000 traces, it's more than likely that there is at least one mistake there. With a bit of bad luck, that could cause the system not to boot at all, which means we don't have anything to show.
Some traces could be too close together for the production process, therefore causing short circuits.
Maybe the powerlines are too thin, or similar.
There are TONS of things that can go wrong. This is the first prototype of a very complex PCB, guys