For starters, it smacks of idolatry.
While there is certainly nothing wrong with loving our fellow humans (indeed, that is the second of the new commandments!), we are to love God (Jesus) first. The Catholic "obsession" with Mary (and, to an extent, other saints) might seem, especially to an outsider, to infringe upon this priority.
Indeed, appearance is another important aspect. When a Catholic seems to be honoring Mary before God, whether that's truly the case or not, that appearance gives the wrong message to non-believers, non-Catholic believers, and potentially even other Catholics. Christians are admonished several times in the epistles to be wary of anything that may cause a brother to doubt, especially things that are not needful... and reverencing Mary is not needful.
Mary is not our Savior. Only Jesus Christ is our savior. Thus, another answer is shown already in the title of your question; it's a distraction. Now, if a little Marianism truly brings you closer to Christ... well and good. But if one continues to devote their attention to a (dead) human rather than to God, that attention is ultimately misplaced, and one ought to repent.
Mary is also not our intercessor. Praying to Mary (or to saints), rather than to God, is deliberately distancing one's self from the God one claims to love. Why would someone who loves God ever want to pray to someone else, rather than to God directly?
Although the argument is often made that we ask other, living humans to pray for us... that is clearly a different case. First, if we're doing so primarily, rather than as a supplement to praying to God directly, it's clear that we're doing something wrong. Second, asking for prayer from a living person encourages fellowship and fulfills a secondary function that God has commanded of Christians "looking out" for each other and sharing their burdens. It works to our benefit in a way that talking to people no longer living in this world can't and won't. This is an important point; the purpose of asking others to pray for you is not to "gang up" on God, it's to share your burdens with those who can help. God can help, because... well, because He's God. Living people can help because we interact with them in this life. Those who have fallen asleep have no physical presence (and many Christians believe they have no ability at all to influence the present, physical world) nor do they have God's omniscience.
Another point... why care about Mary, and not about Joseph? If you've never given Joseph a second thought... you might want to think long and hard about why. You may say "no one knows Jesus like Mary", but surely Joseph is a close second? If you aren't giving Joseph at least half as much attention as you're giving Mary, if you aren't giving the apostles (who were with Jesus continually during His ministry), you probably need to seriously look at why you're holding Mary in such reverence. Yes, she was chosen by God to fulfill a very special purpose, but so was Abraham. So were Moses and Noah.
Claiming that Mary is "alive" is irrelevant. If our criteria for reverence is those who have (eternal) life in Christ, then Mary is no more worthy of our devotion than Abraham, Moses, David, Matthew, Mark, Peter, Paul, the one thief crucified beside Christ, or your Christian grandfather.
What of the claim that Mary is the "queen of heaven"? What of it? "Thou shalt have no other gods before me," declares the LORD (Exodus 20:3). At best, we are still commanded, clearly, to put God first. Moreover, the scriptural basis for this claim is questionable, and many Protestants reject it. Where scripture does mention a "queen of heaven", God has this to say about the dwelling of those who worship her: "Behold, my anger and my wrath will be poured out on this place, upon man and beast, upon the trees of the field and the fruit of the ground; it will burn and not be quenched." (Jeremiah 7:20)
That said, it's no more sinful to respect or study Mary than to respect or study Sir Isaac Newton, or Saint Augustine, or Saint Paul, or one's parents (Exodus 20:12). But we ought not pray to any of these people. We ought not let our respect or study get in the way of worshiping God or studying His Word, lest we find ourselves putting them before God. And we ought to be careful about giving the appearance of doing so, lest we cause others to stumble.