1. What you do doesn't matter, it's the person you're with. I guess one thing you could say is that things just happen to go well, and there's no negativity.
2. When the person you're with annoys you, has bad conversation skills, is racist, kills babies etc. and yet is totally into you and will not take any hint that the day is over, even if you're being forthright about it. I think that would be the worst thing for me. Also if they start crying and say you're the only reason they're living etc. And then you get stuck in a room with them somehow. Maybe an elevator. Effectively any kind of situation where you find yourself stuck with someone despite completely hating it.

Also, I guess being stood up.

3. I honestly don't know. I don't linger in the past and/or spend much time reminiscing. xD I know of dates I've had that were amazing, but for me holidays become a big long date, or a sequence of consecutive dates, and they're far more enjoyable. The road trip along Great Ocean Road (between Adelaide and Melbourne) was particularly awesome.