I've been reading a lot of fail lately, of which a great deal is based on a total misunderstanding of Blizzard's stated design with regards to raid difficulty over time. While I'm not here to comment on the merits (or lack thereof) of that design, I'm getting awfully tired of seeing people whining and boasting who have clearly not read any of the Blues explaining their intentions. So I made an ugly graph to clear some things up and, hopefully, succinctly demonstrate Blizzard's stated plan with an arbitrary difficulty scale:
As you can see, Heroic mode receives few to no nerfs in difficulty over time. Of course there may be bugfixes and adjustments which may alter the difficulty slightly - I chose not to represent this. Every major raid tier added is immediately preceded by a drop in difficulty of the then-current normal mode raid tier. Allegedly this is so that new guilds are realistically capable of catching up in content, but it also throws a bone to the less skilled guilds. The prestige associated with normal mode raiding (whatever you assign to it) will naturally drop, but that associated with heroic raiding should stay approximately the same. Of course the heroic rewards will eventually lose their prestige as people become geared from higher tiers and the earlier content becomes more and more amenable to facerolling.
There is one bit of guesswork here: I'm not entirely sure how much more difficult each raiding tier is going to become, relative to its predecessor. To make the graph a bit more clear (at least to me), the changes in heroic difficulty from tier to tier are exaggerated somewhat, while the changes in normal difficulty are compressed. Also, the final tier of content (I took a guess here and assumed two more after Firelands) may or may not receive nerfs under this plan by Blizzard.
Final note: As I stated above, I'm not here to argue for or against this scheme. For the sake of full disclosure, I'm not even subscribed at the moment and have no stake in the nerfs - but the reasons are financial rather than game-based.
Sanity addendum: Read this before posting