Too much backtracking in the endgame

This is the main issue with the endgame IMHO. You are forced to do a lot of backtracking because:

1. You gotta find all those rare mobs and very often they unveil somewhere on the opposite side of the map
2. Map layouts are simply bad, they are hard to traverse in a linear way, so you often miss certain league mechanics like a Breach hiding somewhere in the corner
3. The safest strategy for maps is to complete it first (rares) and then attempt league mechanics - so once again you gotta backtrack a lot

People generally don't like walking simulators. It's boring.

PoE1 had exactly the same problem in fact, but nobody noticed it since we had the movement skills.