None of them / all of them. You are asking for a subjective answer.

Luther's theses were points of academic debate. Each one is an argument against or helps construct the debate against the sale of indulgences. Which ones were nitpicks? Well, none of them are in the sense that they all are valid ways of arguing against the system of indulgences. In another sense, they do seem very repetitive to our modern ears, perhaps giving the rise to the notion that Luther was nitpicking.

Some context might help shed some light on what Luther might have intended. About a month before his famous 95 theses, Luther published for debate 97 theses which addressed issues he had with scholastic theology. In many ways, this is the underlying theology that gave rise to many of the institutions of the medieval church with which Luther took issue. These theses didn't gain much traction, the debate around this issue was limited to the university in Wittenberg.

So, when Luther sat down to write his 95 theses, one month after his 97, he likely thought of this as an academic exercise. He likely was under the impression that they would be debated by a handful of academics at the university and that would be it. Luther wrote the theses in Latin not German. In short, Luther used the language of academics and theology rather than the common tongue. This gives us some idea of his notion of what he intended the 95 theses to be.

What made this situation different was the fact that Luther was attacking, on theological grounds, a revenue stream for the Roman church. With the threat of a lucrative source of money in the air, the Roman authorities sought to quash the theses and bring Luther to heel as was their MO. The problem was that controversy bread curiosity. The Latin theses were translated into German and distributed via printing presses. This made Luther's ideas accessible. At that point, it wasn't the church verses a rogue monk and professor but rather a more complicated political issue.

So, how many of Luther's theses are nitpicks? Well, depending on your perspective, it might be most. But, considering it was an academic exercise exploring all the possable arguments that could be used, even if it's nitpicking that's what he was doing in the first place.

Source: Justo Gonzalez, The Story of Christianity.

This is kind of an impossible question to answer. The Disputatio or 95 Theses aren't really ninety-five separate criticisms of the Church, like you'd think from the common title. The Latin title is "A Disputation on Declaration of the Power (Force, Efficacy) of Indulgences," which is what the ninety-five points wind their scholastic way around.

The genius move of building a point-by-point argument like this, is that no individual thesis seems that controversial. Like (I use Wikisource's translation):

Christians are to be taught that he who gives to the poor or lends to the needy does a better work than buying pardons;

Who would disagree with that?

And yet, the sum total impression of the theses is that the specific manner in which the Church has mobilized its indulgences campaign cannot help but violate cherished, fundamental dogma. Thus, when Luther writes his major contentions, like:

We say, on the contrary, that the papal pardons are not able to remove the very least of venial sins, so far as its guilt is concerned.

[...]

We say, on the contrary, that even the present pope, and any pope at all, has greater graces at his disposal; to wit, the Gospel, powers, gifts of healing, etc., as it is written in 1 Corinthians 12.

they don't jump off the page, but rather flow naturally out of the previous.

From there, he can question Church doctrine from the point of view of the righteous protector of the Church from within:

This unbridled preaching of pardons makes it no easy matter, even for learned men, to rescue the reverence due to the pope from slander, or even from the shrewd questionings of the laity.

To wit: -- "Why does not the pope empty purgatory, for the sake of holy love and of the dire need of the souls that are there, if he redeems an infinite number of souls for the sake of miserable money with which to build a Church? The former reasons would be most just; the latter is most trivial."

The theses are a fun text to wind your way through and see how Luther builds his criticism to a crescendo--as an insider and sacred protector, not as an outsider throwing rocks at the wall.

Could you be a little more specific as to what you mean by "nitpick"? I assume you mean "criticism that, while valid, wouldn't require Luther to leave the Church"?