Stuff. There's always more of it. That's one thing I rediscover every time I move. There's always more stuff.
Some of it is in the form of physical possessions, though I've done a decent job of whittling those down to the essentials, plus some keepsakes. Okay, plus some crap I just didn't have time to go through. And that's because there's so much of it!
But there's also stuff to do. Make sure I have everything. Clothes, electronics, food for travel, tax information for next year. I've done my research, but there's always more things to think of. At this point, I hope anything that I haven't thought of yet is just not that important, or I would have needed it by now. It's not like I'm going to get to the airport and say, "My passport! That's what I forgot!"
No, it's going to be something like a USB cable for my phone. I'm pretty sure it won't be that, though, because I'm not taking my phone.