For me, it's the knees. When everyone is leaned back you can't even comfortably use a tablet to read, while I can comfortably sit upright for a few hours (of course taking a walk from time to time). The person in front reclining their seat forces me to either manspread into the seat on the left or right (if I'm not on the aisle) or stick my feet in the aisle and getting in the cabin crew's way as they move back and forth.
For me, it's the knees. When everyone is leaned back you can't even comfortably use a tablet to read, while I can comfortably sit upright for a few hours (of course taking a walk from time to time). The person in front reclining their seat forces me to either manspread into the seat on the left or right (if I'm not on the aisle) or stick my feet in the aisle and getting in the cabin crew's way as they move back and forth.