A new expansion, which will wipe the end game raiding instances from the map due to the fact that character progression skips the levels. Talents are wiped because of complete revamp of the talent system, which in turn will effect skills and builds.
As in earlier expansions, the gear you get from normal quests is superior to the end game tier gear from the earlier expansion's raids. The precious Icecrown Citadel crafting recipes become a collectors item more than anything.
What bothers me really is the difference in content, especially the difficulty of the content to the two distinctly differently geared groups: the ones with Emblem gear and the ones with ICC epics. Emblem gear is the group who do the dailies and do not raid, thus lacking the cutting edge gear in general. Maybe a piece here, another there, but still lacking the majority of BiS'. The raiding group has been there, done that, gotten the epics and set is either full or one piece short.
Will the difficulty of Cataclysm be challenging enough for the better geared people, or will they just fast forward through it to the end game (and get bored before everyone else joins it)? Or will the difficulty be much higher for the less geared group, so that they really have to work for the end game in the end?
Or will it be as easy and simple as Northrend: you get so far exceptional gear in greens that the content itself becomes trivial.
That would be a disappointment.