The day of Christ's return will be an assault on our minds, spirits, and bodies. It will inspire "the fear of the Lord" but, hopefully, it will not be the first time each of us feels that fear. We need that fear.
This may seem odd. Fear seems to be a response to something bad. It is a natural response, but not just to things that are bad. Fear is a response to something significant to our well-being, something that our mind recognizes is important to notice (or "take heed" of, as Jesus put it). Holy fear is a recognition of the uncountable importance of God in our lives and, as such, it is also a way to honor and respect Him. God is so much greater than us, so much more powerful, that it will be impossible not to feel fear in His presence.
There is another aspect to Christ's warning, one that I think fewer people will notice right away.
"take heed... lest your hearts be weighed down with... cares of this life"
How often we dedicate our worry to the wrong issues. From Christ's words, we can see that daily anxieties are meant to be managed. We must live our human, earthly lives, but we should not allow day-to-day problems to completely fill our hearts that our heart's attention is taken from a focus on God.
We are warned by Christ not to let daily worries distract us from what is truly, eternally, more important. We are reminded once each year that we come from, and will return to, dust. Our bodies and this world are temporary. The good news is that the problems of this world, then, are also temporary. Everything from terrorism, disease, and hunger, to stubbed toes and that jerk who cut you off in traffic will pass. Paul wrote that love - charity - never ends. But terrorism ends. Disease ends. Hunger ends. Even traffic, long lines, office politics, and bills end. Love never ends and God's love, especially, never ends. It will be waiting for us when we're ready, when we're done focusing on all those other things. If we cannot pull our focus away, God will do it for us. That is the purpose and promise of Christ's warning in the Gospel.