Initially I was a bit disappointed that we still didn't seem to have any trouble two-manning things. Wrath instances were never particularly hard, but I recalled the normal ones being at least a little bit of a challenge initially, especially when you were coming into them with sub-par gear. But no, even with mid-level Outlands gear, the two of us cleared everything up to Drak'tharon Keep with no problems.
Old Kingdom was the biggest disappointment to me personally, though not because of the difficulty. First off, they redesigned the quests to guide you straight past the optional boss, à la Gnomeregan. Not just that though, they also removed the little bonus quest you used to be able to get in the cave with the mushrooms. Mustn't reward people for straying off the main path, oh no! It was a funny quest too: if I recall correctly, you gathered all these "interesting" looking samples from the elementals and then the Nerubian you handed them to gave you a bit of an odd look because they were essentially elemental poop and worthless. The worst thing was the final boss though, as they changed the insanity mobs into generic red blobs that didn't seem to have any abilities whatsoever! Was that mechanic really that confusing as it was? Killing lookalikes of your party members was what made that fight fun! Le sigh.
When we hit seventy-five halfway through Drak'tharon Keep, I put a Glyph of the Treant into my third minor glyph slot and spent the rest of the instance bouncing around like a loony. It was so nice to be an ugly little tree again! I still remember the "save the trees" parade I observed at the end of Wrath. And I have to admit that making it a minor glyph is actually a great solution, as you can keep the look if you want it, or drop out whenever you feel like it, without incurring any practical penalties either way.
In Halls of Stone we had a funny moment when I DCed and died during the Brann fight, and thanks to talents and self-healing, pet tank managed to stay alive long enough for me to not only come back online but also run all the way back into the room where he was fighting. During the NPC conversation afterwards I also had the embarrassing realisation that I've been spelling vrykul wrong for a whole five years - for some reason I always thought that the r and y were the other way round ("vyrkul").
The Oculus was the next instance that turned out to be a major challenge, as our gear was crappy and the drakes only had the absolute minimum amount of health. After a couple of failed attempts on Eregos we eventually managed to get him down by having pet tank circle-kite him on a red drake, spamming his main ability to do damage and get rid of the whelps, and occasionally using his dodge (especially whenever the boss enraged). Meanwhile I sat on top of the boss on a green drake and alternated between focusing on dpsing him and casting heals on the red drake as needed. With the kiting the damage on the drakes was manageable, though it was a very long and slow fight with no bronze drakes to do proper dps.
Trial of the Champion was also pretty tough, and not just because the jousting was quite a drag with only two people. Argent Confessor Paletress did absolutely nasty damage, especially to me, since she does a lot of non-aggro-based attacks and I was the only target that wasn't the tank. I'm not sure I would have been able to survive being feared around and smited as much had I been playing any other healing class, without the druid's heals over time ticking away whenever I was incapacitated.
The ICC five-mans were interesting as well. Again, the bosses in there had a lot more mechanics than previous ones, and I basically had to deal with everything at once since there was nobody else to do so. I always had to kill my soul fragment on my own on Bronjahm; I always had mirrored soul on the Devourer of Souls. On the last boss of Pit of Saron, I was almost constantly incapacitated or transferring damage and healing to the boss, which made for a very long and touch-and-go encounter. Halls of Reflection actually defeated us initially, as we didn't quite have the dps to make the fourth wall, but we came back once we had upgraded our gear a little and made it without problems after that.