What will Christ's Judgement be based on? Are you asking how Catholics understand it -- or asking in the spirit of the college student who wants to know about everythng, "Is this going to be on the final?"
Yes, how Catholics understand it.
Briefly, love of God and love of neighbor -- they really do encompass all the commandments. We believe that Church is the surest guide in helping us to cooperate with the grace of God to grow in love. We believe that, in free will, we can reject that grace -- in fact, we do reject it every time we fail and sin. Enough failures can add up to a total rejection.