A few others have said their biggest mistake was a 'who'--mine was too! Lucky for me it wasn't DP. She and I have a few Disney trips under our belts. We disagree sometimes but agree on elements that seem crucial to me: how much time and money we devote to WDW trips, what kinds of planning go into it, and what to do if we disagree.
After getting back from our May 2015 trip, a (then dear) friend said going to Disney was a lifelong dream, and we all agreed to plan a trip together for November. There were warning signs that things wouldn't go well (most notably, said friend didn't seem to have any interest in the actual planning--we would suggest restaurants, activities, try to talk about hotel preferences, and she just agreed to everything before quickly changing the subject) but the trip was a bit of a disaster. It's what I imagine travelling with a sullen teenager to be like, except she's an adult who would never explain what was wrong, only pout and passive aggressively complain about things on her phone to her boyfriend at home. Long story a little less long, we were constantly changing plans to suit her but nothing seemed to make a difference. Most nights either DP or I would cry from frustration--one time I actually broke down crying at dinner I was so frustrated (and I'm not a crier!).