"HaYom haras olam, hayom ya'amid b'mishpat kol yetzurei olamim." At first glance the two halves of this statement have little to do with each other - there is no intrinsic reason that the anniversary of creation as opposed to any other day must be the day of judgment.
The Ar"i is medayek that the tefila describes Rosh haShana not as the day of creation, but as the day of hirayon, conception. Rosh haShana is not in fact a celebration of creation alone, because, as the Ba'al haTanya's asks, if we were celebrating creation then why does Rosh haShana not occur on 25 Elul, the start of creation? Our focus on 1 Tishrei can be explained with the distinction made famous by the GR"A between malchus and memshala. Memshala is the autocratic imposition of dominion on subjects; malchus is the willing acceptance of a ruler by a group of subjects. The first 6 days of creation were days of memshala, where G-d imposed his will on the Universe to bring about the reality of creation. The creation of man on 1 Tisrei celebrates Hashem's malchus, the creation of a being uniquely endowed with the free choice to accept his Creator through his own volition - this is why we stress over and over in our R"H tefilos the word and concept of melech and malchus. Rosh haShana celebrates creation as "haras olam", a world conceived and pregnant with potential, but not a finished product until man uses his free choice to bring about the malchus of Hashem. Precisely because the world is "haras olam", entrusted to our care to complete, "hayom ya'amod b'mishpat", we are called to account for whether we have lived up to that charge.
Wishing everyone shana tova, and thank you all for your feedback and comments through the year! Hopefully we will be able more learning together in the coming year.