Ohhh that's tough. I don't think I could choose one, but if I had to choose a specific aspect, it would be Pooh Bear and Christopher Robin's friendship. That just made the whole thing so childlike and wonderful.
To be honest, the entire atmosphere of WtP - getting lost in this magical land where there are no grown-ups, no boundaries to your adventures other than the limit of your imagination, and the way it always seemed like this eternal escape from real life, into a place that will always exist in your mind, so long as you want it too.
I really want to go to Ashdown Forest, the place Milne based the books on.