Isn't John the Baptist's humble statement, "the thongs of whose sandals I am unworthy to untie," a bit overstated? Are we unworthy to serve Christ? If so, why? If not, why not?
Not overstated at all, because of our sin, we are completely unworthy to serve Him. However, His sacrifice has covered over our sin. Also, what else can you do except serve the One who died in your place. There really is no other option once you truly come to that conclusion.
How would our lives be different if we were truly filled and flooded with the Holy Spirit?
If we were truly filled with the Spirit we would do what Jesus did, think like Jesus thinks, say what Jesus said, and love with Jesus' heart.
We may be afraid of "fire-of-judgment" kind of language, but what should our response be to warnings of terrible judgment? We should be thankful that we have been given a warning. It is certainly better than not having one!
How good a change-motivator is the warning of future judgment? What other motivators might people respond to in addition to this? Or better than this?
I agree with Hanks that many people these days have "no awareness or fear of any future judgment". I think that everyone does have a longing and void in their heart when they don't know God. They know something is missing and try to fill the void with other things. If we are able to help them see that pattern in their life, how they are seeking something that is missing. Then help them identify the missing thing (Christ) and the purpose He provides us, sometimes that might motivate people to seek Him.