There’s a concept in gaming called time to kill, or TTK. This number starts off at a baseline and usually gets smaller the stronger you get. Makes sense, you swing harder, shoot faster and all that. Designers increase the challenge by either increasing the number of enemies, changing tactics or simply buffing the enemy hit point pool to no longer be linear.
D&D is the perfect model for this. Baldur’s Gate kinda pushed this to the limit where you could die to 3 wolves if you were too low a level but get 1-2 levels above them and they didn’t even matter anymore. 4th edition moved away from simply throwing the kitchen sink at you in terms of numbers and instead added challenge with stronger enemies. Still, a level 20 character is practically a god and that makes it a real challenge for a DM to put an enemy that makes it hard for characters to progress since you need to make your own god.
MMOs have a disconnect here between the leveling game, the grouping game and the end game (or raiding). As you level, it can be argued that your power increases and TTK drops drastically. TTK in WoW is 2 or 3 attacks after level 50. Expansion packs have to reset that TTK though, since there has to be some challenge for players or why bother. While at the tail end of Cataclysm you could kill 10 enemies in 10 seconds, Mists turned that into 20 seconds. Group content is similar where when it’s fresh, you are likely to die. When you are powerful, I’ve seen tanks simply walk up to the bosses and ignore everything else in the zone.
Now we get to what is considered acceptable TTK. I think WoW is a bit low for my tastes, at least in the single player game. There’s no strategy, just 2-3 buttons. The Secret World is way too high (cue meme) at 30 seconds or so. Fights become annoyances where there’s just a meat wall with no impact on strategy. I like the SWTOR model, GW2 to some degree, where you get to use 5 or so skills and get a rhythm going, or at least an understanding of class mechanics.
There’s some comfort in knowing that I have control over TTK and that if that number fluctuates, I can improve my play to address it. If random buttons have the same impact as smart button presses, then I just zone out.