Palace of the Dead (PotD) floors 101+ are great, but one aspect is difficult for the wrong reasons. The rest of the game can be played in ~30 minute blocks. Even savage raids let you change group members between fights. PotD alone requires the same group of people for several hours. It locks the fun behind a fight against your day planner.

Additional checkpoints after floors 100 and/or 150 would help, lowering the time investment for each new run. Allowing you to change group members would also help, so your run doesn't grind to a halt when one person can't make it on day 2. At the very least, some extra data slots would let you hang onto in-progress runs awhile longer until the group members' schedules can align once again.

The difficulty curve and balance of the higher floors is really well done, but many players hardly get to see it simply due to scheduling issues. Since PotD is now somewhat outdated, I think it's time to relax the restrictions and let more people enjoy it.