Generally you will lay out an artist's statement for them to judge against - when you articulate your intentions/goals, someone can tell you how well (in their opinion, of course) you communicated what you set out to. Somewhat subjective, but what can you do - every potential buyer/consumer/critic of your art will have a subjective opinion as well, so you get used to them.
Then there's of course the technical end which is generally rather easy to judge (fairly) objectively.
Because art isn't merely about "expressions, man" and it isn't as subjective a lot of people would like to believe. Making art is not a series of accidents. It's not magic. There's discipline involved, there's process involved, and execution is actually pretty easy to objectively observe.
Okay. Well I'm sure you know about the 3 sections of your portfolio, but you'll basically be judged on your thought process most of all. The quality section probably examines both concept and technique since it's your "best" work, but the breadth section will be judged based on how you have applied the elements and principles of design to your work, and since your concentration is going to be based on a central theme the judges will examine how well you have adhered to that theme, how cohesive your work is and what problem-solving strategies you used in each piece in terms of your theme.
They're judging your portfolio for exactly what you described: technique, composition, etc. They don't care what you did but how you did it.
Also, creativity doesn't exist in a vacuum. If they were judging what you say they're judging, it's not based on "I like this/I don't like this" but rather "what was the idea, who is the audience, how is it communicated, was it communicated effectively?". Just because something is creative doesn't mean it doesn't have to live up to a standard. I mean, yes, you're free to make whatever you want, but don't be mad when people say it doesn't make sense or they don't like it. If something is put out there, someone will judge it.
Yes and no. Say for instance I want to communicate a story about rebelling against a fascist regime. I can choose any medium I want, but how I use it has to make sense. If I decide to write a comic and the entire thing is about a guy who plays tetris, no one is going to understand my message. It would be ridiculous for me to get upset that my idea was missed because it wasn't clear in the first place. That's not to say there's no place for metaphors or subtle references, but that's where more judging comes in (does the metaphor make sense, can people other than those with PhDs in English understand it, etc).
Believe it or not, I pulled that example out of my ass but then I remembered someone actually did almost the exact same thing very well. It's a film/animation set to the theme of Tetris documenting the rise and fall of Soviet Russia: [link]. It was a new and different way to communicate an idea, but it worked because it made sense. The comparisons were clever and they were clarified through the visuals for those who might not have understood it.
Basically what I'm saying is there's nothing wrong with novelty and creativity, but only very rarely are those two factors strong enough to stand on their own.