1-Old Blue. People who don't get lost any other place get messed up on Old Blue. For some reason it's very easy to get mixed up on the summit and head down the same way you came up.
2-Indian Names. Generally, hikers don't dwell on the differences in native place names until confronted with the four and five syllable linguistic marvels that one encounters in Maine. Try reading Potaywadjo, Mooslookmeguntic or Piscatiquis the first time through. Took you a minute didn't it?!
3-Moxie. Any other place on the trail, a pond and a mountain with the same name would be close to each other. Not so with the Moxies. Moxie Pond and Moxie Bald are almost a hundred miles apart. To add to the confusion, Moxie Bald is not on Moxie Mt but on Bald Mt. next to... you guessed it... Moxie Lake.
4-Tree-conomics. Those squares where the mountainside changes color tell the story. The timber industry is a big part of Maine's economy dating back to colonial times. To this point, except for the occasional power line, you've only had to share the trail with other recreational users. In Maine, the stakes are higher because it's people's livelihood.
5-Baseball Bat Shelters. I believe there's only one left. This dates back to an earlier time when the first hikers would cover the floor each year with pine boughs. Definitely pre Leave-No-Trace.