Spiritual and Ethical Issues in the Bamidbar Stories by Dr. Meir Tamari
"The People Murmured" 
"In the scales of the impartial justice of G-d the slightest error of the great saintly men are equated with the worst sins of ordinary people, so that the great leader Moshe is buried outside of the Promised Land, just like the generation that did not want to enter that Land" (S. R. Hirsch). "It was just that Aharon should die like those who worshiped the Eigel that he had made, and that Moshe should be denied entry into the Land just as those who listened to the Spies that he sent" (Abarbanel).
Throughout the generations our commentators have contended as to what actually constituted the sin of Moshe and Aharon at the waters of Meriva; a sin that is referred to four times in the Torah. How it is connected to the obligation to speak to the stone, and why the price for disobedience had to be non-entry into the Land? Why was Moshe commanded to take the staff with him if he was only to speak and not smite the rock? How are we to understand that they failed to sanctify G-d's Name by not speaking to the rock? What does the entry of Israel into their Land have to do with water and speech?
A Chassidic Master notes that in Sh'mot G-d tells Moshe to smite the TSUR which is the most powerful form of stone, while in Bamidbar he is told to talk to the SELA that is the softest form of stone. When Israel left Egypt they were rebellious, stubborn and stiff necked like a TSUR, and so needed a leader with a strong and powerful hand. However, 40 years later, the new generation was trusting and obedient, so that gentle talking was sufficient to guide them. Moshe should have known how to lead this generation differently, but since he still could only beat and scold, he was no longer suited to lead them into Eretz Yisrael.
This idea of a flaw in leadership is the crux of the Rambam's comment, and following him, many of the Mussar teachers, who see Moshe's failure to control his anger as the major cause of his error.
"Moshe's whole sin lay in erring on the side of anger and deviating from the merit of patience when he used the expression, 'hear you now you rebels'. G-d censured him for this; that a man of his stature should give expression to anger in front of all Israel when such anger was not called for. This constituted a Chilul Hashem since he was the model on which they could aspire to find their worldly and their spiritual happiness. Furthermore, from his behavior, they had certainly concluded that since Moshe was G-d's shali'ach, this anger meant that He was angry with them when it was not so" (Rambam, Sh'mona P'rakim). "When Moshe, after 40 years, saw himself directed to the people with the staff of G-d again in his hand, it hurt him grievously to think that he still had not won the confidence of the people and their trust, so in the bitterness of these feelings he forgot his orders and spoke words of reproach ['hear you rebels'] and in passion smote the rock" (S. R.Hirsch).
Since the Torah does not mention Moshe's anger and we know that Aharon was never guilty of anger but rather always sought to bring peace, Ramban sees the sin in another light, one that fits with G-d's saying, 'since you did not sanctify My Name'. We must remember that at Horev G-d had told Moshe that He would appear before him and the people of Israel above the rock, so that there it was clear to all that the water came as a miracle from G-d and not from Moshe and Aharon.
"[In Bamidbar,] Moshe made the mistake of saying to the elders, 'shall we bring you forth water' instead of saying, 'shall G-d bring forth water', as in all the other miracles where the authorship of G-d was made quite clear. Here, the people of Israel may have been misled into thinking that Moshe and Aharon had drawn forth the water by their own skill or actions. Therefore they failed to sanctify His Name" (Ramban).
"Moshe and Aharon thought that Israel were rebelling against G-d and so were unworthy of the highest form of miracle, that wrought through speech alone. They had no confidence that G-d would fulfill that which He had promised and so were guilty both of rebellion and of trespass. Later, the people of Israel recognized the supernatural source of the water and sanctified G-d's Name in the valley of Arnon when they sang the Song of the Well; 'well that princes dug, a gift from the Wilderness, water that went up to the heights' (Bamidbar 21:14-20)" (Sforno).
Sefer Ha'Ikarim [R' Y. Albo] sees Moshe and Aharon lacking in the faith to invoke a miracle that would split the rock and bring forth water. Had they done so, G-d would have surely fulfilled the words of His prophets and wrought a miracle that would have indeed been a Kiddush Hashem. He notes the Kiddush Hashem caused by Yehoshua at Giv'on when he did not wait for a command but initiated his call to G-d, a call that was answered when G-d caused the sun to stand still so that Israel could complete their battle.
"Eldad and Meldad (Bamidbar 11: 26) prophesied, 'Moshe is to die and Yehoshua will lead us into the Land'. Moshe's whole leadership was one of open and public miracles but when they would enter the Land those miracles would cease and Israel would have to live by the hidden miracles of normal everyday living" (Shem MiShmuel). "When they left Egypt, water, like food and shelter were given to Israel by revealed and public miracles. However, now, prior to their entry into Eretz Yisrael, the people had to be taught that in that Land, when they needed rain and water they would simply have to use the power of prayer-speech and G-d would grant the miracle. By smiting the rock instead of speaking to it, Moshe failed to teach them this lesson and he thereby forfeited the right to enter the Land of Israel" (Ha'ameik Davar).