Since I see so, so many people crying about Return to Karazhan being "rehashed content", I thought I'd take a moment to point out a few examples of what a rehash is, and a few examples of what a rehash is not, since people seem to not even know what the word means.
- Zul'Gurub 5man. Some of the same bosses, but with completely new mechanics and new encounters.
Not a rehash.
- Zul'Aman 5man. Literally the same bosses with the same mechanics as the raid, just scaled down to 5man.
Rehash.
- Deadmines update. Same location, but with updated bosses, completely new encounters, new trash, etc.
Not a rehash.
- Timewalking content. Literally the same exact dungeons with the same mechanics, just scaling the players down to their level.
Rehash.
- Ragnaros in Firelands. Same lore character, completely and entirely new encounter.
Not a rehash.
- Level 80 Onyxia released alongside ToC. Same exact encounter, just scaled up to 80.
Rehash.
- Nefarian and Onyxia in Blackwing Descent. Same lore characters, but entirely different encounter.
Not a rehash.
- Level 80 Naxxramas. Pretty much the same exact encounters, give or take a few changes made to accomodate new class mechanics.
Rehash.
Return to Karazhan. Same location for the first half, only four of the eleven bosses being the same as the previous raid, but all of the bosses having new and/or updated mechanics.
Not a rehash.
I consider something to be a "rehash" when a majority of the content is the same, or extremely similar, to the previous iteration.
I don't consider it a rehash to take the same lore character and give it a new encounter with new mechanics. If Return to Karazhan was truly a "rehash", then it would just be the same exact instance but scaling us down to level 70, like Timewalking does. But it's not.
Now, these encounters are not COMPLETELY new and original; Maiden, Moroes, Attumen, Curator...they're still pretty similar to the original version. But different enough that they're not truly "rehashed". The encounters are reminiscent of the originals, but different enough that they aren't just a lazy copypaste.
Even so, the second half of the instance is completely new bosses and new areas, so....yeah. Calling Return to Karazhan "rehashed content" is just a massive grasp at straws.