I have heard a thousand speculations about what it is that hinders us. Habits. Sin. That weight - we must get rid! No! Read the Bible in context! He has spent 10 chapters talking about how useless and what a hindrance the LAW and religious system is! He is saying that they couldn't obtain it through that system! You have got something better - now obtain perfection in Christ Jesus and throw off what THEY had and what hindered them and don't carry on your back the weight that couldn't get these people of faith to where you have got in Christ Jesus in the New Covenant!
That is NOT what is hindering you! It is that very condemnation of the Law that is hindering you! We are not encouraging sin but when you are hindered - then you probably will sin a whole lot more! He has just given a whole book on faith. "They did not enter because of faith".
What is this sin? It is not generic sin but specific sin - it is the sin of unbelief in the goodness of God and in the goodness of our high priest".
Rob Rufus - "Hebrews 6:4-6 - Grace and Glory Conference 2010" - Hong Kong.