The importance of being specific
Big-tent "mere creation," a.k.a. Intelligent Design, is unscientific — not because it was inspired by religious dogma or because it involves "non-natural" causes, but simply because it is so vague as to predict nothing at all. No possible discovery could be inconsistent with the hypothesis that living things were created at an unspecified time or times by unspecified methods by an unspecified and possibly undetectable creator or creators with unknown motives and possibly unlimited power. Likewise, the unadorned hypothesis that all living things came into existence by the mechanical operation of natural laws (possibly including unspecified laws which have not yet been discovered) is not a scientific theory. As useful as they are in politics, big tents have no place in science.
Hypothesize a purpose.
The various features of living things — body parts, organ systems, behavior patterns, coloration, and so on — usually have clear functions, but living things themselves do not. It's easy to state the purpose of a lion's sharp teeth, a giraffe's long neck, or a swallow's migratory instincts; much harder to guess the purpose of a lion, a giraffe, or a swallow.
From a Darwinian point of view, each organism is "designed" for one purpose and one purpose only: to ensure that copies of its genes continue to exist in one body or another for as long as possible. All other evolved functions, whether directly or indirectly, serve that one ultimate goal. The problem with this from a creationist point of view is that it means the phenotype exists only to perpetuate the genotype, and the genotype exists only to perpetuate itself. The purpose of a lion is to prolong the existence of lion DNA; the purpose of lion DNA is to keep copies of itself in existence by creating lions. In short, it means that an organism has no purpose which transcends the organism itself — which seems inconsistent with the idea of having been designed and created for a purpose. To what end did the designer create the living things we see? — and "He created them so that they would exist, preferably for a good long time" just doesn't cut it as an answer to that question.
Maybe the creator's purpose is so far beyond our understanding that we can't even speculate about it. Or perhaps the various features of the biological world have so many disparate purposes as to make any theory indistinguishable from ad hoc rationalization. If so, fine; it doesn't disprove creationism, but it does make it harder to establish it as a legitimate scientific theory. Some religious people will find it impertinent and perhaps even blasphemous for humans to speculate beyond what has been revealed about God's purposes. All I can say to that is that if the act of making a hypothesis is objectionable to you, you have no business doing science; if you want to give your creation myth the prestige that comes from the label "science," you have to be willing to subject it to the scientific method.