In reading Hebrews I encounter two things that stand out - how it teaches things that the East would seem to have some problem with (wrath), and how Luther was highly concerned with how repentance seemed to be downplayed in a place like Hebrews 6.
This can all be rightly understood when look and see how Hebrews moves through the ideas of wrath, sin, and sacrifice.
Hebrews is clear, over and over, that God exercises wrath. It cites the Old Testament in 3:11 - "As I swore in My wrath, 'They shall not enter my rest.'" God's wrath desabathizes man. Chapter 4 builds on this quote, and it lingers throughout the first 10 chapters - we are told that Vengeance belongs to the Lord, that there is punishment for sin, to the point where Chapter 10:31 can read, "It is a fearful thing to fall into the hands of the living God."
This is not some Eastern "Oh, if we don't love God, we just don't handle His presence well" - that is far, too far, an anthropocentric approach - one that is quite foreign to Hebrews.
What we see in Hebrews is this. Sin = wrath. Wherever there is sin, there God will be wrathful. Period. End of story. Don't try to soft peddle this - People sin, they get wrath. They don't know the ways of God - they get wrath.
However, there is another idea that is strongly attached to sin in Hebrews. Sacrifice. Sin is sacrificed for. And that sacrifice (and I say one, for the point is that all sacrifices point to the one Sacrifice of Good Friday where Christ is the propitiation for our Sin) does away with sin -- and where there is no sin there is no wrath.
If sin - then wrath. If no sin - then no wrath. There can only be no sin in Christ. The only place where there can be no wrath is in the sacrifice of Christ which takes the fullness of all wrath for sin -> it is only in this sacrifice that sin is remembered no more and that wrath is stilled. Wrath exists - it exists upon sin, the sin that He who knew no sin bore for us to the Cross.
But then, what of Luther's worry about 6:4ff? No repentance? I'd pair this with 10:26 - for if we go on sinning deliberately after receiving the knowledge of the truth, there no longer remains a sacrifice for sins, but a fearful expectation of of judgment and a fury of fire that will consume the adversaries.
To reject Christ is to reject His sacrifice for the propitation of sin. It is to say, "I do not want Christ to have my sin" - it is to claim it away from Christ unto yourself. So be it. Take your sin - but where there is sin, there is wrath, and you will have it in spades.
And there is no other repentance, no other place we can turn to. Hebrews 6:1 -- Therefore let us leave the elementary doctrine of Christ and go on to maturity, not laying again a foundation of repentance from dead works and of faith towards God.... for it is impossible to restore again to repentance... This is a statement not of the unforgivablity of sin, but that there is no other sort of repentance by which man can live - and if one rejects the faith of Christ - they make the death of Christ as nothing, they crucify Him to their own harm because His crucifixion becomes not the death of their sin but something else, something less.
If you look other than to the death of Christ Jesus for your forgiveness and deliverance from the wrath of God, you will find neither. There is no other turning to which you can turn, no other penting towards which you can repent yourself.
Thus, Hebrews 11 - come, let us fix our eyes upon Jesus, the author and perfector (the "It is finished"-er) of our faith. Thus do not forsake the gathering together (in Christ) as some of you have become accustomed to.
The entire book of Hebrews is really an explanation of the fact that Jesus is the Way, the Truth, and the Life - that no man comes to the Father but by Him, by His atoning sacrifice upon the Cross, and that apart from Christ there remains only for the sinner wrath. Thanks be to God, we have been made to Christ Jesus our Savior!