What great questions, sasark.
I agree with cubanmiamiboy, I'd say the most important technical sequences are the main pas de deux in Act II, the "4 little swans" (they dance all in a line holding hands until the final release - look for complete uniformity and staying on the music), the black swan pas de deux in Act III and the Act I pas de trois.
I think the 32 fouettes are very important. Not necessarily doing 32 per se, but the technical mastery and staying in one place (not traveling all over the stage while doing them) is what counts.
What really wraps it up for me, is a dancer who can master the dichotomous challenge of playing Odette and Odile. Frequently, you see a ballerina who can embody one really well, but not the other, and you end up feeling unfulfilled as a viewer. It's not just about technique.
Which company are you seeing?