I will go out and buy Wrath of the Lich King later today, didn't want to queue up for it at midnight. And then I'll install it, and start playing, doing quests, and trying to get into dungeon groups. And in a few levels my current epic gear will be replaced by a mix of green and blue items with better stats. The process in which new items in a MMORPG make old items obsolete is called mudflation, and it will apply to just armor, but also some tradegoods in WotLK. I sold all my gems, as I'm expecting the gems of WotLK to be both better and cheaper than current rare socketable gems.
For characters currently still in green or blue gear, Wrath of the Lich King will bring a huge step up very quickly. Even early quests and the first dungeon run at level 70 in Northrend are likely to yield some gear upgrade. Most people will enjoy that, because a quest that yields some better gear is always more fun than a quest that just yields vendor trash.
Players with characters in epic gear are likely to have somewhat less fun. For a few levels the quests and even dungeons won't bring any upgrade to their gear. And once you get gear with better stats in the mid-70s, it will probably look less good than those epics you had. Apparently Blizzard made the typical Northrend armor somewhat more stylish than the Outland quest rewards were, which often made you look like a clown. But I'm pretty certain that the really good-looking shapes and textures will be reserved for the level 80 epics. Fortunately I'm not much of a slave for fashion, I'll wear whatever has the better stats.
I don't think that mudflation can be avoided with the current "add 10 levels" model for WoW expansions. If level 71 greens were only slightly better than level 70 greens, and much worse than level 70 epics, then the players who are already wearing the epics wouldn't get any better gear until much later in the expansion, maybe even only at reaching the level 80 dungeons. The big step up in item quality also allows to level the playing field: Players currently in epics enjoy an advantage of better stats and thus faster leveling for a while, but the other players will catch up on gear quickly. After all, Blizzard has to think of new players starting World of Warcraft only now, and of Death Knights, all of which will arrive in Northrend without ever having had the opportunity to gather level 70 epics.
Mudflation isn't necessarily a bad thing, although there is a risk of demotivating a certain type of players. Some players see the value of raiding in getting the epic gear, so making that epic gear obsolete somehow diminishes their achievement. But for those who can't bear to wear the same gear as everybody else, there is always the possibility to rush to 80 and work on the next set of epics. I'm afraid that is what far too many players will do.