This drastic change in how levelling is done makes me feel that TBC is just tacked on to the vanilla game, and the fact that walking through the Dark Portal also obsoletes a ton of old level cap content doesn't help. I have the same feeling about WotLK, it obsoletes all the TBC endgame content, but at least the questing philosophy hasn't changed.
So WotLK feels tacked on to TBC which in turn feels tacked on to vanilla. And now Cataclysm is not too far away, and it will probably feel tacked onto WotLK again. Yet I still suspect I will enjoy the third expansion the first time around at least. I am odd in that regard.
I'm saying this because I actually loved my first trek to 80. My second one not so much. And my third... gah. I don't think there will be a fourth unless they decrease the experience needed from 70 to 80, something I believe they will do eventually, they already did the same to 1-60 and 60-70. It won't change the fact that Northrend levelling content is damn annoying (to me) though. At least there will be less of a slog to get through it.
I hope Blizzard will make Cataclysm content more replayable, especially the revamped 1-60 content. I am pretty sure they will take away what I like about vanilla levelling, the spread out quests approach, in an effort to get people to endgame even faster. Personally I'm not too fond of doing it this way, but I have no power to influence that decision. What I do hope though is that they make at least two different paths you can take so even the second new alt will find him/herself with new content to see. Up to level 60 at least.
At least there will be more heirlooms to speed up the process even more. I just wonder how high the experience multiplier can get? 50%? 100%? Maybe they'll even add heirlooms with a minimum level of 60, these bringing the total up to 200%, like RAF. If they do this then they might even allow trading of heirlooms between the two linked accounts so that the bonus remains until level 85.
That's an easy way to deal with boring content, by making sure you spend as little time as possible in it.