You're being harsh on Ivanchuk.  He's a one-off, a one man celebration of marching to a different drum.  Chess professionals do not need to fit into someone's idea of how they are supposed to play or behave (within the law, of course).  Nobody can say that he didn't give it his best, or that he faltered against the top two players.  I grant you he would make a strange world champion, but top-level chess needs more guys like him to keep things interesting and off balance.

I don't think Carlsen stumbled under pressure.  I think he was exhausted.  And yes, so was Kramnik.  But until the final round, who was more solid?  Kramnik.

I don't want to speculate on the WC.  I honestly don't know what is going to happen.  Normally you'd think the more brilliant player would prevail, or the one with more stamina.  But brilliancy comes in different flavors and stamina can be measured within a single game or a series of games.  Before this match I thought "if Carlsen wins the Candidates he'll clean up against Vishy".  Now I am not so sure.  I can almost picture Carlsen taking an early lead, holding it for several games, and then cracking.

This tournament was not just a sample of one.  This was IT.  The only route to a world championship, against the most formidable players in the world.  Maximum pressure.

My point is that this was a moment when someone's mettle is tested to the absolute max, and how they perform under those circumstances tells you more about their inner reserves than any number of other situations.  And since they come around so infrequently, you will never have a good sample size.  Indeed, the peculiar high-stakes, double-blind situation that existed in the final round of this tournament was probably unprecedented in the annals of world championship-level chess.  And what was revealed?  Even the very best are only human.

> (By the way - I wouldn't say Carlsen collapsed. His main mistake was to use too much time - which is indeed often a sign of nervousness.)

I have a slightly different, though similar, explanation here, and one quite different from Nelson's (with which I would normally agree if the tournament situation was a little bit different from how it actually was).  I don't think that it was nervousness--I believe that both Carlsen and, to some extent, Kramnik made judgment errors here in using too much time...because each wanted to see what was happening in the other game before making major decisions on how to try to direct his own game.  In so doing, each let his clock run too low without making any major commitments, with the result being that in the distractions and in the eventual time trouble, Carlsen turned a win into a loss and Kramnik turned a draw into a loss.  Their opponents, who had no such distractions (though Chucky was very short on time compared with Kramnik until the last few moves before the time control), were better focused on their games during the latter stages and were able to sweep up the full points.

Regarding one of your other points:

> [Ivanchuk] would probably have become world champion if he had the mental strength of Karpov or Kasparov

I definitely agree with this--indeed, I would go further in that I believe that if you take all of the top players on their best days, Ivanchuk would be stronger than all except for Carlsen (though I still do think that Carlsen would have top honors here).