Yet another set of comments
Strange Leveling/Tier Progression: when I first started playing, I did notice a difficulty peak around levels 15-25, which decreases after that, then becoming more difficult again after level 70 or so. Now being a grizzled veteran player, I found that using vendors and gamblers more aggressively early helps get better items more quickly, helping to overcome this early peak.
Cave-Ins: I play mostly hardcore, and sometimes need to play with the sound off, so I got used to watching for those little puffs of smoke that signal a cave-in, and getting away with little or no damage. My level 51 and 33 HC characters are still around because of it.
Suicidal NPC/Escorts: For escort quests, after spotting the NPC, you can clear the path to his destination, then go back and accept the escort. You'll almost never fail an escort quest this way. Rescue quests are different. Sometimes they die before you get there, or are down to a sliver of health and are surrounded by monsters. Not much can be done there, but if you see an NPC has a fighting chance, you can give him food and/or a health pot giving you extra time to clear his monsters away. Not foolproof, but does increase your chance for success.
Money Drops: I sort of like the multiple cash drops. I think dead leprechauns use them as a revenge mechanism, since I was killed once by a monster that trailed behind leprechauns that I didn't see due to the large number of cash piles on the ground.
Mana-Steal Monsters: one of the enjoyable things about this game is the uniqueness of the monsters, and part of the fun is figuring out how to deal with each one. Yes dark elf mages suck if you're a mana user, but not so much if you're not. Being attacked by a pack of amorphs sucks if you're a melee character, and not so much if you're a mana user with AOE.