Metaphysics is the investigation of the world outside of us via our minds rather than via our senses. It ultimately amounts to a denial of the world as we see it because it always claims that there is always something higher and somehow more "real" that lies behind what we are capable of sensing. Because we cannot sense this extra layer of reality, the correct organ for its investigation is held to be the mind.
And metaphysics, by and large, is dead.
Metaphysics died under the assault of modern science. Scientists hold that the only things that can be truly known about the world are those that we can sense and record and so prove. Science aims to produce truth-claims of the sort, "Under X conditions, Y will occur". Such claims have an incredible plausibility because they can be repeated. The metaphysicians never managed to create anything of this sort; nothing they ever said could be proved in the real world. And as we humans always must remain part of the real world and grounded much more firmly in it than in the realm of thought, the victory of science was inevitable.
The most basic and oldest metaphysical idea of all is the idea of God. It was hence no coincidence that when Nietzsche said that God is dead he really meant - as I have explained in that writeup - that metaphysics was dead. Cicero said that the idea of the existence of gods is "engraved in the minds of all men", and Hannah Arendt has suggested that the idea of God stems from our ability to think. What she means by this is that God - and metaphysical concepts - stem from our ability to think beyond what we can actually sense, and to imagine perfection; in politics, we call this utopia, but in spiritual terms this perfection is God. The fallacy is to imagine that just because perfection exists theoretically in our minds, it also exists out there somehow as a part of reality.
Because these things cannot actually be found in reality, metaphysics had to hold they could be demonstrated by reason. But metaphysics proved self-defeating because of the strength of the claims it made about what could it could discover. Plato used to say that a grounding in mathematics was essential to a study of philosophy, and for millenia the illusion persisted that the products of our thought could have the same factuality as the fact that 2 + 2 = 4 or of the existence of some object that you and I can both perceive and agree exists. But the products of rational thought alone, like God or the Platonic forms, can never achieve the level of verifiability of scientific truths.
Modern science shattered this illusion by redefining the notion of truth and claiming that only that which can be proved in reality through empirical investigation can be held to be true; all is to be doubted and investigated. And because metaphysics could never produce experiments that could be replicated, the trains of mental reasoning it produced looked entirely irrelevant to an understanding of the world.
The problem faced by metaphysics lay in the fact it confounded two types of truth. Science has as its correct field of investigation the empirical world that we can sense, but it has nothing to say about ultimate questions of meaning. This is one type of truth. Science deals with what exists without ever asking why it exists - what made it exist, what its purpose is, what will ultimately cause it to cease to exist and why; this is another type of truth. Claiming that we could know the answers to these questions in the same way that we can know that science's products are true was the greatest error made by the metaphysicians, and is why the rise of modern science so completely discredited them. This was part and parcel of philosophy's descent into ignominy, and it was highly dangerous because it simply removed the most fundamental questions facing us about the world and meaning from the field of human enquiry.
In the last century, some philosophers attempted to move "beyond metaphysics" by redefining its search for truth. Foremost among them was Martin Heidegger, who said that metaphysics should be seen as a form of poetry. Poetry is the perfect example of a thing that gives life meaning and vitality without ever trying to prove anything; nobody would claim that poetry is useless because it cannot be proven to be true. Heidegger went on to support Nazism, causing the indignant outrage of posterity; yet he was merely the most systematic representative of modern, scientific man, who shed all his prior beliefs that could not be proven and plunged into the task of finding meaning within himself, like a poet. Once we accept that meaning - and hence how people should live with one another - cannot be proven, there is no recourse but the retreat to a commitment to an arbitrary value system.
And this is why are playing with fire.