I started playing group content again in SL, and during that period I've gone from fumbling around with my casual guild doing +5s, then clearing Nathria normal, getting KSM in s2, joining a new guild and getting Curve in Sepulcher and now poking around 20-21s in S4 (although not sure if that's actually harder than KSM in s2).
I've done this as both dps and tank, and my takeaway is a) raid tank is booooring b) the impact of dps in dungeons (and raids) is vastly underrated by a lot of people and c) people mix up difficulty of playing your role with how hellish the pug experience is.
I don't like pugging much, and mostly do it if I get at least a friend or two on discord for running keys. It seems mostly fine but occasionally hellish in different ways depending on role.
In the first degree, tanks have a huge impact on keys, because they set the pace. That's intrinsic to the role, the one that pulls sets the pace, so unless you get a hunter with MD to pull *everything* (or backseat drive your friend via discord), the tank sets the pace, and the pace is crucial to a smooth dungeon run; fast enough that you make the timer, not so fast that things fall apart.
But once you've found the pace, DPS has a very large, and often unnoticed impact. Mediocre dps can hide, sure. Deal their damage, refuse to interrupt, get healed through mistakes. You won't get very far with that. Good dps, though, the ones that carry 15-20 keystone and are actually capable of pushing past 20, will hit their interrupts, do every mechanic, take far, far less damage than the guys blaming the healer in 15s, and do up to double the average dps while doing it. Doing all of this at the same time, while maintaining high dps, is difficult.
DPS is more relaxed insofar as that less is expected at around 15s. You'll muddle through in time, the healer will cover their mistakes etc etc. But playing DPS properly has a huge impact, which is usually not noticed in lower keys.
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DF M+ changes address this directly in a sense, with new M+ pools each season. I picked up tanking this expac at the end of S3, and even though I knew every dungeon as DPS I still felt the pressure having to learn all the routes and stuff. That part was made easier when S4 hit and everything was new.
Also, most people are content to let the tank set the route *anyway*. It's the flip side of the tank being responsible for the route: you run whatever route the tank wants, because you've asked them to set it. Used to skip the big guys in Stormheim? If the tank pulls them you shrug and kill them, no biggie. You lose a bit of time from a 'perfect' route, but most routes are perfectly viable*.
*big exception being the bridge in upper kara. Fuck those mobs.