"Grievous". What's that?

We get some examples. Grievous is used to describe wolves set to destroy the flock, plagues, sores on those receiving the mark of the beast in Revelation, the whirlwind of God's judgment in fury on Israel. As far as I can see, grievousness is that manner and degree of pain and suffering wrought as a consequence evil upon the doers of evil; it is not (again, as far as I can tell) the suffering of those who obey God. That is a different sort of pain.
Now, there can be some overlap by perception; the chastening of the Lord seems presently grievous and not joyous, says Hebrews 12:11. The passage goes on to state that what that kind of pain yields, though, is the "peacable fruit of righteousness unto them which are exercised thereby" (KJV). Most of us can attest to the sense of God's afflicting hand for the purposes of correction; I dare say, though, that few (if any) of us can truly speak of grievousness of any particular part of walking with God. Why? Because the hardships which the Bible painstakingly explores in the lives of the obedient are experienced with the hope of that same peacable fruit that those who are "exercised" or made stronger and abler by chastisement. Grievousness offers no such hope (again, as far as I can tell).
So, when I read that His commandments are not grievous, I don't quite get to the point of believing that through God's love and the Holy Spirit that obedience is not "hard" nor "full of suffering". Getting beheaded or thrown to the lions or sitting in a North Korean prison for conducting corporate worship qualify under both headings as far as I can see, and all of them are consequences of obeying His commandments and of knowing God's love and the Holy Spirit in that particular way of excruciating hardship.
Dietrich Bonhoeffer, the famous theologian who was hanged in a Nazi concentration camp, wrote from his prison cell that "You have granted me many blessings; let me also accept what is hard from your hand." He also wrote, "I can no longer condemn or hate a brother [or sister] for whom I pray, no matter how much trouble he causes me. His face that hitherto may have been strange and intolerable to me is transformed through intercession into the countenance of a brother for whom Christ died." And, "As a Christian I am called to treat my enemy as a brother and to meet hostility with love. My behavior is thus determined not by the way others treat me, but by the treatment I receive from Jesus." (He is a most compelling guy, so, here's another ---

.) "His disciples keep the peace by choosing to endure suffering themselves rather than inflict it on others. They maintain fellowship where others would break it off. They renounce hatred and wrong. In so doing they over-come evil with good, and establish the peace of God in the midst of a world of war and hate." Now, say what we will about Bonhoeffer's views on war and pacifism, but we can't disagree with his experience and we'd be hard pressed to biblically dissent from these statements which are clear on the point that suffering while obeying is a distinctly and truly Christian way of life.
All that's not to be dismissive of your observation, beckrl. I just wanted to point out that while obedience is not grievous, it is hard and the hardship is due to God's love and the Holy Spirit shaping those of us who do suffer the worthy experience of being made like Jesus in suffering.
Human government bears the same relation to hell as the church bears to heaven. (David Lipscomb, On Civil Government, 72).