Noteworthy are the following:

1. Putin could have altered the Constitution to stay on as President, but did not.  I think it would have been very detrimental to his credibility if he had done that, but I'm not convinced that's the only or primary reason for his decision to step down at the end of his term.  I'm inclined to agree with Vilhelm Konnander's comments:

At the news conference Putin said: "Throughout all these eight years I have toiled like a slave in the galleys, from morning till evening and, have done so with the full devotion of my strength." This is most probably a very sincere statement, and is also in line with what Putin has previously said repeatedly. Also, people working in the Kremlin has let it be no secret that the Russian president has been quite tired and weary of his duties in recent years. So, being a slave to power does not in Putin's case have to be a fixation to power, but an actual slavery of duties. Still, media have failed to see this.

So if he has not stayed on as Prime Minister as a result of some lust for power and megalomaniacal paranoid control-freakism, why has he?  Ok, listen, I am not an expert.  Obviously.  But I do read a lot, and there appears to be a great deal of consensus among those for whom russophobia has not compromised their capacity for critical thought.  And that is : he doesn't feel his work is done.  Especially on the domestic end of things.  He's re-established Russia's place on the international stage, but inflation, corruption, gap between the rich and poor, birthrates, etc. remain problems, and I suppose he imagines he's best suited to address them.  We'll see.  But what it boils down to is that he knows his legacy -and that's what all of this is about- hinges on the success of the country.  Ensuring Russia's success, and that no one comes along and tramples on, reverses, ruins whatever accomplishments or "reforms" he's implemented = ensuring his legacy.  

2.  Everyone said Putin, when personally chosen by Yeltsin, would be a puppet of Yeltsin's regime/people.  He was specifically chosen for his loyalty, after all.  Well... we saw how well that worked out, didn't we? :)  Fact is, and I re-iterate, there is no sense in trying to predict the future, esp. when it comes to Russia.  

"Pretending that you already know the answer when you don't is not actually very helpful." ~Migeru.