(With patch 4.0 changes, my Arcane blast spell crits for more than 25K damage, which means that I can easily kill just about any level 70 boss in the 30 seconds that the Mirror Image spell keeps them busy. On top of that damage, AB now automatically applies the Slow spell debuff. This is actually more effective for single target crowd control than Frost is, as the Slow spell was balanced with the now-outdated assumption that players were going to be spending 20% of their time recasting Slow if they wanted the effect to stick.)
Though this tactic did allow me to cross a number of items off my to do list (all but the last boss of Arcatraz; if any mage has soloed this, I'd be curious to know how), in many cases it was so easy that the dungeons were no longer fun. Playing as a frost mage requires carefully juggling aggro and freeze effects. Playing as Arcane involves spamming Arcane Blast until everything is dead. There were only a handful of encounters (notably the Raven boss Anzu, who I actually had to switch back to Frost to deal with) that were still in any way interesting with a character as powerful as a modern day level 80 Arcane mage facing level 70 heroics.
The Issue With Difficulty As A Motivator
Ghostcrawler, WoW's head system designer, posted a lengthy philosophical piece last week explaining how they felt that healing in the Wrath era had become overpowered, much as my mage is now overpowered for many dungeons that I was working on. Cataclysm is designed to challenge healers with more limited mana regeneration to address various (and significant) issues that resulted from the old status quo. The problem is that, unlike my choice to solo old instances because I'm bored, these changes don't occur in a vacuum.
Group content in World of Warcraft (and most other holy trinity MMO's) is designed to require 20% or more of players to play healers. This constraint on class requirements for groups has created a situation where large numbers of players who do not want to play healers are doing because someone has to (and/or because their personal wait times are lower as a result). When someone is doing something that they want to do, you can appeal to their sense of reason that increased challenge will make their victory more enjoyable. When someone is only doing something because their guild needed another healer, increased challenge only makes an unpleasant task even less enjoyable.
I believe the crab when he describes the problems that resulted from healers who never ran out of mana, and it's entirely possible that the game the crab envisions for Cataclysm would be more enjoyable than the one we have today. That improvement is reduced to an academic point, though, if the majority of players (who choose DPS) have dramatically less access to actually playing the game because the guy with a healing off-spec no longer thinks he can handle healing the instance and decides to queue with all the other DPS instead. It's going to be very interesting to see whether Blizzard sticks to their guns if the increased difficulty leaves the highly popular random instance grind unplayable.