I mostly liked the 'end bosses' of the new zones, the problem is, save for maybe Hyjal, after it was over, everything ran right into a brick wall. Beat Skullcrusher the Mountain? Good Job! ...Now what?

Plus, to me that feels like that would be isolating people a bit *too* much. At least in open world you can theoretically come across someone and team up if you wanted to. Now no longer needing to group up to progress at all?

I've thought about this a lot, and I'm sure the devs have too. The problem is: How in the nine hells do you balance a solo boss encounter to be fun and challenging for, say, a prot paladin, while also being *doable* by, say, a holy priest? Or perhaps that's a bad example, since I doubt there are very many solo-only holy priests, and they probably aren't worried about balancing the solo content for raid healers ...how about...what's a squishy glass-cannon class? Fire mage?

Some specs (blood DKs, prot paladins, possibly some flavor of lock or hunter) can solo Marrowgar in ICC and find it a fun challenge. Other specs (holy priests, holy paladins) have a hell of a time doing Tol Barad dailies by ourselves. I don't see how you produce fun, challenging, interesting solo bosses unless you create 30 different versions of it designed to challenge each spec. Or at least five versions - one tanking, one mdps, one rdps, one raid heals, and one tank heals.

Nice suggestion! Actually, this has been implemented in some other games already, for example DCUO. You get to run "solo instances" with a boss at the end. You can also enter those instances with more players, but if you do, there is more trash, and I believe the bosses get harder to kill.
Anyway, that game has different roles too, and I have run those instances as a tank, DPS and controller (kind of like a designated buffer/CC role), and it does work. Granted, its a bit harder if you're a glass canon (like a clothie DPS), but then again, if youre a tank, you might not die quickly, but your dmg output is lower, so it might take you longer to actually kill the mobs.
I do believe that that would be possible to implement into wow, especially with dual spec. Play a holy priest? Respec to shadow for those instances. They shouldnt be comparable to a 10 man raid in their difficulty.

In the game I mentioned, the instances were harder with certain specs, but they all were beatable, as some bosses required you to use a certain tactic, using the environment to bring them down, or you could just burn them down.

I know some people will now complain and do the good old generalization ("Casual players are unable to communicate"), which is funny as I remember how a lot of raiders had to face equally unfair generalizations back in the day.
Fact is... a lot of times, a casual player might not have the time to get a group together, might not want for an LFD, or might not wanna put up with the attitude you often find in those. I for one gave up on tanking instances for PUGs due to the behaviour I saw from a lot of people in those.
Suggestions like this one arent supposed to say "Stop 10 man raiding, lets all solo everything", its meant to be an addition to the "core game" and the endgame-side that offers something more for casuals, because even if some people hate to hear it, there are a LOT of casual players in the game, and giving them something like that wouldnt be a bad idea, unless someone thinks just because he has more time to play, he owns the game and the developer's time. =)

So, nice suggestion, and it has been done, and I do think it would work without taking anything away from other play-styles. Just my opinion though

...actually, the more I think about it, the better I can imagine it. I somehow doubt it WILL be implemented, as it'd take a lot of development still, but... regarding different specs, it might actually be a great way to learn how to use more of your abilities, and maybe also get you interested in raiding eventually.
Imagine youre in such a solo-instance...

You're a tank - Try to wear the mobs down, use your defensive CDs to keep yourself alive.
Youre a plate DPS - Kill the mobs quickly before taking too much dmg (fury or arms) or use your heals efficiently (pally)
Clothie DPS - Use your CC, whether its frost nova, polymorph, MC, fear, your pet, whatever
Rogue - Use sap and stealth to skip some trash or make groups manageable
Healer - Maybe something like in the LK-encounter or the Ursoc the bear god encounter, where you pick the NPCs role and support him... he tanks or does dmg, you do dmg and keep him alive?

Anyway, nice stuff to think about imo =)

The NPC helper for the quest to kill Ursoc in Grizzly Hills worked well, I think. You could tell him to tank, dps or heal (of course, he was a druid). Some of the current "sim-raid" end quests in Cata are also fun, and there's a lot of quests in the starting Forsaken campaign that pair you up with helping NPCs.

Perhaps a system that would provide missing companions if you were solo, but leave them out if you were in a group?