Regarding Feanor and the Definition of Greatness
S.H.O.

(In 2nd response to Mark S.)

As a telling exercise, you can scan several dictionaries. None of them will include, in the definition of "greatness", wording such as "a tendency toward moral decision-making", or "quality possessed by one who chooses to perpetrate no evil", or "characteristic belonging to no one but a selfless champion of what is righteous and just, as seen from a Western, Judeo-Christian viewpoint", etc. What you will see are synonyms like "might", "mightiness", "impact", "power", "influence", "force", etc. The word "greatness" carries with it no judgement calls made regarding personal character in any dictionary I can find.

Again, ask yourself this question, "Were Melkor's deeds strictly evil?" And if you answer "yes", which you must if you have read the Silmarillion, and you still believe only those of pure heart can be great, ask yourself another question, "Why did Tolkien repeatedly state that Melkor was the GREATEST being to reside in Ea?" You cannot continue to argue that Fingolfin, or Galadriel, or whoever, was greater than Feanor due to distasteful and disqualifying flaws in Feanor's character, until you resolve this very important dilemma that your side of the argument logically generates. You have to address Melkor's dual status as the mastermind of all evil AND the greatest being before you proceed on the assumption that one cannot be the greatest whatever without being kind, or good, or moral, or worthy of fond admiration, or righteous, etc. You absolutely have to! You cannot drop the condition that those to be considered great, or even greatest, be made from the utmost moral fiber for Melkor, and then hold Feanor to it!

To further my point, take a look at a real life example: Hitler. If the leaders of the world in the `30's and `40's had not underestimated Hitler's greatness, he would never have been allowed to become strong and establish himself by regaining lost German territory, and fortifying the German army. By the time Chamberlain, FDR, et al, realized Hitler was one of the most influential people of all time, it was too late-the evil to European Jews and other Europeans had been perpetrated. Hitler was indeed a great and powerful man, but he was, unfortunately, also a very twisted man. Take a look at any powerful historical figure (Napolean, Alexander the Great, Mussolini, Ghengis Khan, Stalin, etc.) and you will see selfish deeds and appalling decision-making similar to Feanor's. Tolkien was no dummy, he knew how to craft believable characters. 

Forget whether they are good or bad, just concentrate on the weight of Feanor's actions. When doing so, remember that the book was called "The Silmarillion", after the Silmarils that Feanor created. Not "The Fingolfillion"! If the entire history of a people can be summed up in a word derived from the works of one person, then that person's deeds must have had the greatest impact on those people, and none among them can rival that person's greatness. No one had more influence over the fate of the Elves than Feanor. Therefore, Feanor is greater than Fingolfin, or any other Elf.