It wasn't always like this. There was a time when heroics were actually hard. I'm not talking so much about how they got nerfed for speed reasons (gogogogo no time to sit around waiting for Ionar to disperse three times, we get the idea, once is enough), but rather about a time when did them in mostly blue gear (kinda like how Gevlon and co raid ICC now), when the ilevel 200 epic at the end was a highly priced bit of loot. One of our tanks ran Utgarde Pinnacle twenty-six times before the Red Sword of Courage finally dropped for him, and this was long before LFD meant tanks could get a group at the drop of a hat. Back then, that sword was the best pre-raid tanking sword in the game, and even the ones from Naxxramas weren't a huge improvement, at least in 10-player.
I sometimes read about how the design of the Northrend instances is conducive to them becoming an AoE grindfest, and to be honest I'm not really convinced. The harder heroics were not like that in the beginning; tanks simply could not survive the constant high level of damage from a pack of mobs while they were all burned down at once - they had to be taken out one by one in order to reduce the incoming damage to a manageable level, much like in Halls Of Reflection now. People might even have used crowd control(!) to help out. Imagine that!
No, the problem I think is gear level inflation. When the highest level of gear was 213, all was fine, and nobody really over-geared heroics unless they were part of a 25man raiding guild that had Naxx on farm. Then came Ulduar, where 10 reasonably motivated people could get together and kill bosses and get 219 level stuff, or even 226/232 if they fancied a real challenge. Then came Trial of The Crusader, upping the gear level to 232 for 10man (245 for 10man heroic), along with the 5-man Trial of The Champion instance which dropped 219 gear on heroic mode. Aaaand again when Icecrown Citadel was opened: higher item level (251/264) and new 5-man instances dropping loot of the same level as that of the previous raid tier. Add to that the availability of a full Tier 9 Set of 232 armour just from the emblems that drop from the bosses designed for 200 blues, and you can see where the problem is.
Blizzard have got themselves into a bit of a bind here, with their desire to make the latest tier of raiding accessible to everyone. That's all very well, a noble goal, but the side effect has been to make the previous tiers more or less obsolete. The only reason people raid Ulduar, Naxx or Obsidian Sanctum these days is for the achievements, and their associated titles and mounts (or, rarely, just because it's fun, but let's not get into such esoteric concepts here).
So, what's an MMO maker to do? Well, one solution would be to reduce the disparity between the first tier of raid gear and the projected last tier. It really shouldn't be possible in 4th tier gear to output over eight times the DPS you can in first tier. I think the merging of 10- and 25-man loot tables in Cataclysm will go some way towards this, as there no longer needs to be the leapfrog where 10-man gear needs to be better than the prievious tier's 25-man stuff. Reducing loot inflation may also mean fewer loot whores, who only raid for teh shinies. If the next tier's gear isn't vastly better than the current stuff, there'll be less incentive to raid just for loot's sake. Instead, people might raid for (shock horror) the fun and challenge of facing new content. Furthermore, there'll be less of a reason for Blizzard to introduce "wellfare gear" available from emblems: since the differences in gear level won't be so important, it won't be so necessary to have the latest and greatest just to stand a chance in the new raid (although as Gevlon's proved already, that isn't even the case now).
I have no experience with the transition between two expansions, so I'm unsure what to expect when Cataclysm hits.I remain hopeful, however, as despite the problems WoW has now, it's still a damn good fun game and if Blizzard can make it better, I'll be there, trying to bring Deathwing down with 9 or 24 other folks.