I beat Okami a couple weeks ago. Fantastic game.
It took me about 3 months to complete (to be fair, my system was gone for 3 weeks when it was repaired by Nintendo).
Total play time: 59 hours, 57 minutes
I completed every side quest (that I know about) and completed everything to as close to 100% as can reasonably be expected (and with almost no help, to boot). I completed the animal tome, obtained every single collectible and even saved up 2 million yen (w/o selling any treasure!) to purchase brown rage (which was what I felt after learning the move is absolutely worthless). The only thing I didn't finish was the fish tome--I'm missing 3 entries and I refuse to fish any more. I probably also missed a few scattered treasures, although I did get at least one of everything.
Minor complaint: I spent hours collecting those damn stray beads--I got about 85 of them on my own; had to look up the last 15--and for what? An item I don't get to use unless I want to play again? I just spent sixth hours playing your damn game! Why would I want to start over?!
Random thought: I remember reading on the MTV game blog that according to the Nintendo Channel the average amount of time put into this game is a paltry 19 hours--not nearly enough to complete the game. Strangely enough, this is the exact time it took me to beat Orochi the first time. When I returned to Kamiki Village after the battle Issun wouldn't let me leave the village because I hadn't talked to everyone. I spent over a half hour scrounging around and still couldn't find anyone I missed, but the little bugger wouldn't let me leave. (Eventually, I got frustrated, looked at a FAQ and discovered I just needed to try again two times in a row and he would finally let me escape. Boy, was I ticked!) Did this happen to anyone else?
Anyway, my ultimate point is, I wonder if people thought that either A) the game was over or B) couldn't figure out how to get past this part and gave up. That could certainly explain why average play time it so short. It seems like a natural place to stop the game, so it doesn't seem out of the realm of possibility.