The Haunted Mansion is my absolute, hands-down favorite attraction at WDW. I have easily gone through it a hundred times (over many, many years) and I can still always find a new detail.
You are shown into the Portrait Chamber, and there is a moment of pitch blackness at the end of the show there. Nobody jumps out at you, and the (dim) lights come back on after a very short time. You walk through to where you step into your Doom Buggy, and from there you ride through the house. The effects range from agreeably spooky to fun spooky, and as others have noted, there isn't any gore or horror at all. No live characters lunging at you from dark corners, for example.
The scariest effects are the ghouls which periodically shoot up from behind tombstones with a shriek (if you keep your eyes open, you'll always be able to see them coming, so they shouldn't be a shock), and the bride in the attic, which is only scary because it's frightening that they would take the original, subtly creepy effect and bathe her in day-glo colors with a garishly beating heart and just ruin the mood, but maybe that's just me.
Don't miss the ghost under the table in the ballroom, and if you decide to join them, don't forget your death certificate.