On certain occasions, I remember back to when I was a recent 60. Our guild, Cracked Anvil Trading Co., was pretty much all recent 60's. This was before the patch, that changed many of the level 60 instances (Stratholme and Scholomance) into 5-man only instances. You were able to zerg these instances with 10 people, and farming your Dungeon Set 1 armor wasn't much of a problem. Well, our guild, being almost all noobs at the time, set out on our first raid of Stratholme, live side. If I recall correctly, we didn't even have everyone at level 60. I'm sure we didn't have a single purple; and if we were lucky, we had a blue. It was an ugly experience, and I think, after time, we eventually cleared that first corridor... I think. Fast forward a few weeks, and we could clear it no problem. Fast forward some more time and the whole instance became easy. Even after the dreaded 5-man cap patch... it was an adjustment, but it eventually became easy.
A huge part of this "easy" mode for dungeons is basic knowledge. If everyone is familiar with the tricks and strategies to kill Boss X, then it makes the event so much easier. The other part, is the gear that you obtain makes you more effective at your role.
Bring this into current day, and I hear many people say that Karazhan is easy. If you don't believe me, head on over to the official WoW forums. You can read many people screaming about the ease of Karazhan... how it should be on a 3-day reset instead of 7... how it's a shame that there's not a heroic setting for Karazhan. I think people forget that this dungeon is helping people ramp up, that there's a learning curve, that people still need gear from there to make them successful, gear from bosses that are difficult to clear in a raid of full blue armor.
This idea comes up because of a run I did last night. It's only my third time doing Slave Pens on heroic mode. First night was a bear. Second was much more of a breeze. Last night, however, we didn't have a single death to our party. It became "easy". We all knew the pulls or we had somebody with enough knowledge to tell us what to do. We had a skilled tank, healer, and DPS. While there's still things in there that we want, the instance is probably slightly beneath us as far as our level with our gear.
So now when somebody brings up Slave Pens, I can easily say "that's easy", but is it? With other players at my gear level, it certainly is. I'm sure to a Black Temple geared group, Karazhan would be easy. I doubt there's anything really "difficult" in the game. It's just about gaining the knowledge, gearing up, and then eventually, most things become easy.
Even more pointed to me, there are parts of Karazhan that have become easy, but we certainly haven't mastered the whole thing. Even in our own guild, we have people that struggle more with some of the "easy" bosses. It will take time, but I'm sure eventually, we will all find the instance easy.