This is where judging criteria and guidelines come into play.
I've judged a UK aquascaping contest for PFK magazine and found guidelines helpful. I borrowed heavily from the AGA judging criteria.
2007 AGA Aquascaping Contest
I tried to analyse each aquascape with an open-mind, but even then, of course, we are all human and have personal preferences. This is why a panel of judges is important, to make it fairer.