Remember that you'll still be gaining power at every level, and still get new gear that's more powerful every step of the way. The difference is that the giant stat spikes at the start of every expansion will get smoothed out (and the later power gains proportionately smaller; i.e. if you were 150% more powerful at level 85 than you were at level 81, you'll still be 150% more powerful).
So, the problem is that with raiding, each tier of gear needs to be more powerful than the previous one in order to make sure gearing up feels appropriately epic. There also needs to be progression for the players; it's no good if getting gear from heroic Siege is only a small step above normal mode Heart of Fear. Where this all gets jumbled up is when they need to have a catch-up mechanism for players going straight from the end of one expansion to the start of another, it creates a giant hill and everything starts snowballing. It's a problem with no easy solution, since the numbers will always eventually grow larger.
For instance, let's say that you want to make sure each tier is roughly 20% better than the tier before it. That means if the first tier is player power 100, the second tier needs to be 120, the third needs to be 144, then 173, 207, 248, and so on; eventually it spirals out of control. The jumps need to be there when the content is current, but they can be removed and smoothed out when the content is made irrelevant by a new expansion; you can go from player power 100 to player power 130 after five tiers, and "squish" everything between them.