There is the trend in Diablo 3 that the maps are increasingly brighter. Don't get me wrong, there are still plenty of darker maps such as the sewer system in Act2 or the descent through the layers of Hell to destroy the Heart of Sin.
To be fair, Diablo 3 endgame content is Inferno mode. This translates to the game not even starting for many until level 60. The difficulty scaling isn't linear, which is why only 70,000 have progressed into Inferno out of the millions that are currently playing (and only a fraction of that have progressed past Act2). Once in Inferno, the hack and slash aspect is 100%removed, unless you are working the Acts backwards in which case it is only 85% removed (hehe). This is because in Inferno the monster leveling exceed what yours can be. Act 1 the monsters are lvl 61, Act2 they're 62, Act 3-4 63. This means A LOT when it comes to how you approach things, seeing as a normal white mob single hit will register for between 16-40k on a character fresh out of Hell difficulty. Navigation requires a tighter budget of cds, kiting/choke point techniques, cc, damage mitigation, situational awareness, an intimate understanding of class mechanics, and reactivity. Even then, the special mobs have 4 affixes. They're designed to be tougher than bosses required for progression seeing as their defeat is required for endgame content gear. This is for both the nephalem buffs needed for rares to drop from storyline progression bosses and for the gear they themselves drop. This is protect us from people botting for gear as well, seeing as a computer ran player character doesn't stand a chance against one of these mobs. I wouldn't say at any point a Molten, Jailing, Vortex, Desecrating special mob is "easy". I would not define monks as "OP"" when put in endgame content either, seeing as they are melee. Even with top tier damage mitigation tank sets Act 3 white mobs still hit for 6-8k damage each, with each pack having anywhere from 4-30 minions.
The current standings of "OP", supported by both Blizzard posts and forum cries, are currently 1. Demon Hunter 2. Wizard 3. Witch Doctor 4. Monk 5. Barbarian
It's one of those games you can literally run through as if it's Dynasty Warriors until you hit Act 2+ of Hell difficulty... Inferno is where the game began for me.