Typically, the body in a theorem environment begins on the same line as the heading. If the text begins with a list, the first line looks a little funny due to the indentation before the label.
I'm feeling too lazy to figure out how to put in an image myself. I'm talking about the effect in Theorem 1.2 here; compare to Theorem 1.1 at the same link.
I don't want to address this issue by making the text start on the next line (as in the linked question). Instead, I'd like for the first item not to be indented, i.e., the label of the first item should appear in the same position that the first letter would appear if the body began with text.
enumitem, if it matters.