Why is it not allowed to pronounce God's four letter name ywvh? Is there a reason for this? Is it such a great taboo as to be unforgivable?
This is explained by Azariah De Rossi in Me'or Einayim Imrei Binah Chapter 6.
The Light of the Eyes p.149
Thus in all the examples given, the word used for the tetragrammaton has the meaning of lord and does not connote being. This is due to the fact that although the tetragrammaton is written, its letters are not pointed with the vowels requisite for its proper pronunciation. Thus in his letter to Marcella, the translator [i.e. Jerome] speaks of "the four-lettered name which is ineffable." It is not simply as a sign of respect that the name is not pronounced – this is the view of the Christian scholar Pagninus in his Thesaurus of the Holy Tongue – but also because the actual vocalization is not known. Thus the sin of he who "pronounces the letters of the Name" is twofold: he articulates a name which does not exist; and according to his own view, would be behaving disrespectfully regarding something which is even forbidden for human beings, for it is not allowed to call one's parents by their own name. Thus the ban is not on placed on the one who simply reads, which would mean reading in the usual sense, but on the one who meditates. For meditation conveys the sense of something which is veiled or hidden as is demonstrated by the expressions consider my meditation (Ps. 5:2) and May the meditation of my heart be acceptable to You (Ps. 19:15). (Weinberg translation)
The Torah is quite specific about vainly uttering the Tetragrammaton (Shmoth/Exodus 20.6)
6 Thou shalt not take the name of the L-RD thy G-D in vain; for the L-RD will not hold him guiltless that taketh His name in vain.
In Shmoth 3.15 we are told that "the Name" of the Holy One is "a memorial forever"
15 And G-D said moreover unto Moses: 'Thus shalt thou say unto the children of Israel: The L-RD, the G-D of your fathers, the G-D of Abraham, the G-D of Isaac, and the G-D of Jacob, hath sent me unto you; this is My name for ever, and this is My memorial unto all generations.
The original understanding of the third commandment, “You shall not take the name of the LORD your God in vain” (Exod. 20:7), was that one must keep one’s vows when swearing by God’s name. Israel's ancient sages eventually came to interpret this commandment to mean using the LORD’s name lightly or frivolously. To avoid the risk of employing the divine name irreverently, the sages ruled that one should not utter it at all.
It is also listed in Perek cheilek of Sanhedrin as a sin so severe that one who does so (utters the ineffable name outside of the temple) has no share in the world to come. It should be noted that repentance can save even the greatest sinners but WITHOUT repentance one who utters the name would indeed be cut off from the next world.