Reductionism is a perverted tendency linked to the discovery of something that is, often genuinely, good or important. The problem with the reductionist is that, in discovering something good, he wishes to insist upon the exclusivity of that which he has discovered: there are no goods besides the good which he can now claim as his own, since he has discovered or acknowledged it. Reductionism is a kind of pride, a perverse pleasure taken in the goodness of something which is "most important," to the exclusion of all other goods.
Oftentimes, the reductionist correctly identifies something as important, incorrectly identifies it as the most important element of some system or organization. Materialists reduce the real world to matter, because they have discovered at least something good and true about the effect that matter has on the construction of the world. Rationalists, on the contrary, reduce everything to reason and logic, because they have perceived something good and true about the effect of rationality upon thought - sometimes even upon reality. I have also heard one description of existential Thomism which characterizes it as the tendency to reduce all knowledge to metaphysics - because adherents of this school have recognized something true and good about the importance of metaphysics. These are very broad descriptions; there are many kinds of materialists and rationalists and existential Thomists, but among them all, reductionism describes the tendency to identify the good thing which they have discovered as the only one; all else is either unimportant or non-existent.
Sometimes, indeed, the reductionist is even correct in identifying something as being somehow the "most important." One may indeed correctly maintain that metaphysics is the "most important" of all the sciences; but the reductionist consequently tends to reduce all knowledge to metaphysics, so much so that the natural sciences, the mathematics, the arts, etc., begin to find less and less of a place in human knowledge.
Another common example of this latter form of reductionism, which correctly identifies something as most important yet incorrectly reduces all of reality or knowledge to that one thing, is the the form of liturgical reductionism which correctly identifies the Eucharist as the most important and central element of the liturgy, and yet consequently tends to disregard or even discard all other elements, as if they were merely accidental.
Contrary to this reductionist tendency, simply because metaphysics is the most important science does not mean the others may be neglected; on the contrary, it may indeed be necessary to grasp the natural sciences, natural philosophy, even mathematics, in order to grasp metaphysics; that is, precisely because metaphysics is the most important science, it is necessary to have meticulous care for the subordinate sciences. Likewise, precisely because the Eucharist is the most important and central element of the liturgy, all other parts of the liturgy must be assiduously cared for. That which is most sacred or most noble in any hierarchical system - the hierarchy of sciences or of the liturgy, for example - is protected and preserved in its very sacredness precisely by the "trappings" and surroundings in which it is enshrined. A final cause is best attained by the most loving and careful attention to those which are ordered to it, for its sake.
The reductionist might truly discover the Good; but he forgets the Good tends to diffuse and share itself with other goods, expand and extend itself indefinitely - not close in upon itself and make itself smaller.