2.  The government's ability to prove the defendant's behavior. You're only going to wind up at a sentencing hearing at all if the government is able to show what you did; if the government doesn't show it, you're going home. Thousands of crimes go unpunished and even undetected. Thousands more get charged, but unless the government can produce the evidence to prove it, it will not be punished anyway. The government can fail to come up with the proof for a great variety of reasons. Among them (although by no means an exclusive list) are lack of resources, incompetence, and laziness. It is not unheard of for the defendant (and, rarely, his lawyer) to make evidence unavailable by hiding, manipulating or destroying it, or by bribing and/or threatening potential witnesses.

We should not lose sight of this central factor  --  the government's ability to produce the evidence  --  when we talk about a different matter, the standard of proof at sentencing.  For by far the most part, the standard of proof there is preponderance of the evidence.  American law historically has never required proof beyond a reasonable doubt at the sentencing phase. (This accounts for, among other things, the fact that a defendant can be sentenced in part for behavior the jury did not find beyond a reasonable doubt).

In one aspect of the dispute considered by SCOTUS in the Booker decision, the Court had the choice of requiring sentencing proof beyond a reasonable doubt (that is, where the district judge was considering a sentence beyond the statutory maximum, where "statutory" was construed to refer to the statute establishing the Sentencing Guidelines).  But, unwisely in my view, the Court did not make that choice. Instead, it elected to keep the standard of proof where it is  -- preponderance  --  but make the Guidelines "advisory only" instead of mandatory.

3.  The judge.  The judge is the only person in the courtroom who can impose a sentence.  Period.  The lawyers can influence it (see infra), but in 25 years of litigating, never once did I see a lawyer impose a sentence.

As anyone who's been to court knows, the judge has enormous power.  This is especially true with institutional litigants like the US Attorney's Office and the Public Defender, but it is true in every instance.  The judge can dismiss the charge or charges from the getgo, find facts affecting the sentence pretty much as he pleases (taking an appeal from a factual determination is like tilting at the litigation windmill), sentence within or outside the guidelines with a high assurance of no reversal on appeal (there are now more sentences outside the guidelines than within), and, if a statutory minimum is in play that he views as utterly out of line with the defendant's culpability, hold it unconstitutional, see Marbury v. Madison.

Of course the great majority of judges do not view statutory minimum sentences as unconstitutional.  This is not because they're sadistic, stupid or intimidated. It's because statutory sentences have been carefully considered by Congress, and, for by far the most part, the judges applying them are faithful to the Constitution, well-established higher court precedent, and Congress's independent power. 

4.  The statute stating the offense of conviction.  Judges almost always stay within the limits established by the statute the defendant has been found guilty of violating.  

5.  The probation report and the guidelines work-up by the probation officer.

6.  The lawyers.  Lawyers are a self-important bunch, and this fact inevitably shows up in assessments of their role in sentencing.  In my experience, those assessments get a lot of ego-inflation. 

It's true that a shrewd and hard-working defense lawyer (say Judy Clarke, who now has her hands full with the Boston Marathon bomber) can sway a tribunal; this is one reason lawyers are paid more than most other people.  It's also true that the prosecutor has (sometimes powerful) influence by selecting which charge to put before the grand jury (and then to bargain away or decline to bargain away).

But the prosecutor's power is easy to overstate, especially by those looking to put the blame for long sentences somewhere  --  anywhere  --  but on the fellow who committed the crime.  If he's not looking to be shown up for a fool, the prosecutor faces a hard ceiling on what he can allege:  He is limited to charging what the defendant actually did.  

When I was an active litigator, this was taken without controversy as what prosecutors should do:  Put in the indictment a truthful description of the defendant's conduct.  Prosecutors have the option (out of lack of resources, reward for cooperation or just mercy) to charge something less than what the defendant did, but they have no obligation to do so.  

I always thought the job of AUSA was, in a sense, easy:  Find the statute that most aptly fits the defendant's behavior, truthfully describe that behavior to the court, then let the other actors in the system do their job under the law.

N.B.  Because this subject seems to have become so controversial, I would put in a respectful reminder about C&C's rules for commenters, which do not accommodate personal remarks or condescension. The blog is, and will remain, about substance. As to that, commenters are welcome to robust disagreement.