Hi Kristin,
If I had a chance to go to Mickey's Not So Scary Halloween Party, I'd be there in a second! I love it and certainly don't think of it is too childish. The Magic Kingdom captures the mood of Halloween and fall with its grinning pumpkins that light up the windows on Main Street to the fog that drifts down the stairs at the train station to the assorted Halloween music that floats throughout the park.
The Boo To You Parade and Hallowishes are unique to the party, and I think some of the best offerings throughout the year. A quick search on youtube.com for the party or Boo to You parade can help you sample some video clips of the party to see if it looks appealing for you and your husband.
Throughout the parks are trick/treating stations, character meet and greets, and some dance parties that may appeal more to kids and something you could skip without any problem, but maybe the inner kid in you may decide to at least want a bit of chocolate from the trick/treating stations!
Also, the crowds are lighter at the party (especially non-weekend dates), so you can take advantage of short to no lines at the rides. Don't worry about costumes either. Guests will both be in costume and not in costume.