I'm not saying I think Blizzard should make new boss encounters for old dungeons, I just think they should beef up the hit points and damage output of the mobs and bosses. Maybe if the dungeons actually required you to be awake and not alt-tabbed half the time, end-game wouldn't be so painful to new players and we might hear a little less complaining about difficulty.
I don't think making the bosses last longer and hit harder necessarily increases the difficulty of the boss fight, though. The healer just throws more heals, the DPS and tank just keep stabbing away, and it takes more time, not more thinking. To make those fights more interesting and require the brain-power you're thinking of, you need to introduce mechanics, and you need to tune those mechanics to the expected output of a party in that dungeon.
The problem with doing that in pre-BC/BC content is that all of those dungeons have such a huge level-range of players who can get in there that the expected output is wildly variant. There's no way to tune the mechanics when the tank is 5 levels ahead of the DPS but two levels behind the healer. But you can't constrain the level range because that lengthens the time people are sitting in a queue to get into the dungeon. And doing that just stretches out the time between starting a fresh character and getting to the endgame.
Look at the difference in complexity between pre-Cata Deadmines and the Deadmines Remix. THAT is what you'd need to do to all of the pre-BC/BC dungeons in order to force players to get a challenge out of them. And that's a LOT of work.
When a player doesn't even have time to use 5 global cooldowns they learn nothing about the game. Rogues can't even get enough combo points to use their ramp up abilities, let alone finishers. Bosses don't have time to use abilities that require tank or healer cds. Sometimes entire fight elements are eliminated because the boss dies so quickly. Where their was a chance to teach the value of interrupts, or dispels, or positioning, there's nothing.
They would likely learn as much about the game by fishing for coins in Dalaran. And then they get to heroic Lost City of Tol'vir and wonder what happened.
I'm sure Blizz has to contend with the potential of players quitting if the content is too hard too early, but I think their biggest mistake here was removing the entire learning curve. As mentioned a lot of players go from facerolling Gun'drak to getting crushed in (easy) cata heroics.
How about giving heirlooms +10% gain for every alt past level 80 on your account? After 5 alts you'd be earning inordinate amounts of XP and leveling wouldn't be a chore for players who have paid their dues, but also you wouldn't have to nerf the entire leveling process for the sake of a few players.