I've done a fair few day hikes on the Maine section and we quite often rent cabins near it including very close to the 100 Mile Wilderness.
It's tough. The path is rugged - a lot of it involves hopping from boulder to boulder, or walking along exposed roots, or climbing up and down rock faces.
It's unbelievably buggy, and can be very muggy in summer. Every time I hike it I end up with a massive headache.
You're not really likely to be attacked by people or bears - it's very rare. But having said that, the last time I saw a bear while camping in Maine it was not scared of people, and had no intention of leaving until it got some food. Take bear spray or a gun.
It is very easy to get lost, and people do wander off the trail and die.
There are not really many views to look forward to, and the ponds/lakes have all sorts of interesting leeches in them.
Personally I'm not that keen on the AT trail, and much prefer some of the other mountain hikes in Maine such as Borestone Mountain.
If you do plan seriously to hike it, read David Miller's book Awol on the Appalachian Trail. It's much more of a realistic take on actually hiking it than Bill Bryson's book (although nowhere near as entertaining).