Here's my take, there are pros and cons all year:
Avoid peak school vacation times. They will be the most crowded, though they offer the longest park hours, too. If you go during a really uncrowded time, expect your first timer to say, "What are all these empty queues about?" A moderate crowd will more likely impress your first timer a little more. It can be creepy to be in a park that has long, empty queues.
Consider avoiding the winter months (Dec - early Feb), as the weather will be cold. It's no fun to go on a water ride when it's cold. The pools also loose something when it's only 50 degrees outside.
Avoid peak hurricane season. Enough said.
The thing about going in spring is, after a long winter (depending in where you're from) - there's nothing like seeing fresh flowers and feeling the warm air! Certain times are crowed - but Epcot's flower show was really nice. A little bit of crowds is not a bad thing for a newbie.
May through September can be HOT. I dunno...for me, I don't enjoy crowds when I'm really hot.
Late fall has low crowds (nears the end of hurricane season), but maybe isn't as thrilling a time as spring.
For a first trip, I'd avoid the Christmas decorations. Reserve that for a second trip, when you can appreciate the difference. Disney under regular decorations is grand enough, and a first timer adult might find something actually distasteful about combining so much commercialism with such an important relgious holiday (and potentially be offended if they are not a Christian)