What I hate the most are the upgrades, and people, and how the progress and saves work.
First of all, your progress is only saved every 10 floors, so if you get DC'd, or people quit before the boss, or people kick you, it's fine, you just wasted a good amount of your time for absolutely NOTHING.
You only have 2 saves, and fixed parties are locked to the member you choose before the first floor, if you try to get a pre-made they can simply remove you from the party when you're on floor 40 for no reason. Also there's the way to create parties with matched, but 4 people, and then you loose all your poms every boss.
The upgrades, argh the upgrades. They will make it drop from earlier chests with a small chance, but it's awful. First of all people thing they are dependent of your level and want to clear every mob on the dungeon, really annoying. People are bitching that you open chests away from them, but the effects are always global. Also when you get matched with people that need upgrade from the earlier floors they can be really annoying and even kick you on floors ending with 8-9 if you say "but killing everything is not going to help" or "no need to clear rooms to get golden or potions if the key is unlocked". The best part is when you queue with people with good upgrades, but they don't know what they're doing, or they believe in the level = upgrades BS or they want the poms that will be lost in 2 floors.
Tired of being kicked for trying to reason with these people, you try to be nice and explain how things work and they simply kick you and you wasted all your time.
