What is the most common thing that is missed when planning for a hiking trip?
It can be anything.

Leaving at least an approximate itinerary with someone else before setting out on your hike and checking in with them to say you're safe as you come back out.

Aimless has given a good answer.

I would add not believing the weather report...or not being prepared for the worst case scenario on that report.

Packing a map.

We once had a group of 10 dayhikers walk into our camp at dusk, in the wilderness area of the Red River Gorge in KY. They were lost, about 5 miles past the turn they needed to make for the easy loop hike back to the trailhead. They had no map, and no drinking water (or anything to carry it in - I guess that's another thing for the list.) We had multiple maps, showed them where they should have turned (it was well-marked) and invited them to spend the night with us. They said they'd walk back out. We offered them a map, a couple of spare water bottles, and a flashlight (oh - three things) - all of which they refused. That's when I realized that you can't fix stupid.

It's gotten so, on dayhikes, I always carry a couple of spare maps of the park - and usually give at least one away to a misplaced dayhiker.

In the Escalante region of southern Utah we encountered a young couple who were trying to make it to the Golden Cathedral and had no idea what way was up. No map, all new gear and clothing withh the young woman’s fair complexion starting to redden. So I gave them a quick lesson in topography, told them to keep the drainage on their left side and don’t stray as the trail would appear there on down that would lower them into the canyon. Then I gave them my map. We were heading out after several days of adventure. So they had all this new, neat gear but no navigation skills. That’s a huge no no. That is the top skill so you don’t become lost in the first place. I also pointed out a couple landmarks as navigation is a multi level skill. Look as you go. Look forward and backward. Know your terrain. There were very few signs of any trail most of the time with all the sand and slickrock. I hope they made it an had fun while learning a bit of navigation.