I wish I had known that our first trip would not be our only trip. I went into it thinking it was going to be a once in a childhood thing for DD. So we went all out - it was our first big (not on a shoestring budget, staying on a family member's floor) family vacation. We stayed in a deluxe, got the dining plan, went to a character meal every day. If I knew then that that trip would "connect" with DH, and we'd plan to go every 2-3 years after, I probably would have calmed down a bit. we went at a leisurely, but brisk pace. It was SUCH a PERFECT vacation for us, but I wish I had known then that we'd be back just 2 years later. I would have stopped to smell the roses more. Let DD sit and take in the whole parade, not just snippets while DH and I darted in and out of shops. I wouldn't have been so die hard about HAVING to get into O'Hana, so much so, that we ended up with a 9:30 ADR, an hour past DD's normal bed time!
But coming back from that vacation, I remember on our flight back home thinking how I hadn't even raised my voice once that entire week, that DH and I got along exceptionally well (during a time when that was NOT the norm) and that THIS is what being a family was all about - enjoying ourselves together.
Bonus tip: I wish that I had known when I got back that not everyone else would be as enthusiastic about Disney as I was. Not everyone who has ever been had an awesome of an experience as we did and not everyone WANTS to go back. My sister doesn't even get it... and thinks we're nuts for spending $$$ to go back to the same place, stay in the same hotel, go to the same parks again and again.