What does it mean to "covet" and why did it make it on God's top 10 list: the 10 commandments? 

Coveting is defined as an excessive and unrestrained desire with idolatrous tendency.  In ancient Israel, it had economic overtones, a sense of the material.  Bottom line: it is a dark, insatiable drive to acquire (get) things.

Why is this bad?  Isn't it a victimless sin?  No. 

Most obviously it is a form of idolatry.  A horse with blidners knows only one way because of its impaired vision.  So it is with coveting.  One marginalizes God, puts God on the periphery, as s/he exercises an unrestrained drive to get something: more things, more power, more real property.  Coveting marginalizes God and magnifies things, 

Secondly, coveting reflects a lack of one's trust in God's providential care.  Jesus taught his disciples to pray, "Our Father ... give us this day our daily bread ...," and assured them that he had come so that we might have life and have it more abundantly. (John 10:10) God does not promise and then not deliver.  He doesn't give with one hand and take away with the other.  We need only trust, knowing that God, who knows no limits in his giving so that we might have new life in Christ, will also provide for our lesser needs. 

And finally, coveting reflects an unhealthy selfishness.  Eve manifested the first biblical instance of covetousness.  In Genesis 3:6, she succumbed to the devil's temptation that she eat of the forbidden fruit so that she would be like God.  When presented with "my way or God's way," she elected the former, in disobedience to God's rule.  She was at the center of her universe, placing herself on the throne of her life while endeavoring to dethrone God.  Not a good choice, and one that reflects a sinful self-centeredness to it, all to the detrimnet of herself and others.  One could argue that the sin of coveting was humankind's downfall!  

Believing God has a plan and purpose for your life and has and will continue to equip you accordingly, is the antidote to coveting.  Put differently, contentment is the antidote.  To be content is not to be lazy, or passive, or unmotivated.  It is not the antithesis of ambition.  Rather, it is a calm assurance that while endeavoring to be the very best God intended you to be in furtherance of God's unique purpose for your life, you have all that you need to move in that direction.  Look around you.  What you have, who you are, your friends, your resources, your home, your spouse and kids, etc. ... these are gifts from God specifically for you as you live out God's purpose and plan.  There is no need to put the blinders on!  Things have a shelf life.  Love of the world, is not of God.  (1 John 2:15-16)  The love of God brings life and peace.   

So, be content.  Be grateful.  God is faithful to provide our needs.  Like any good ship's captain at the helm, just keep your life in the deep channels away from shoal waters.  In the deep waters of the Spirit, one finds life and contentment. (John 4:11-14)