Hey, buddy, congratulations on quite an honor!
It's a really, really good thing. It also is a great chance to help great games get even more recognition, that they deserve so much.
First of all don't get to caught up in trying to be objective - that's why they have a lot of judges, because they expect them to have different opinions. It's totally ok to have your own personal preference even though you can't quite explain it. That's what games do their audience
Also I'm sorry for some shameless self promotion, but I thought, this might come in handy: http://artisaverb.info/Purpose.html
Best of luck! I'm sure you'll make an awesome judge;)