Nova is a lightning skill, Frozen Orb is a cold skill, and both attacks have great AoE's.
Since there are very few monsters that are immune to both Cold & Lightning, you'll have very strong attacks against almost everything (and your mercenary can take care of the few dual-immunities that you do encounter). As a bonus, both of those skills don't require a ton of skillpoints, so it's (relatively) easy to build strong attacks.
FYI, I recently did a moderately-detailed review of this build (and a few other sorcs) for Baal runs. If you're interested:
http://diablo.incgamers.com/forums/showthread.php?t=683498