In my opinion, it's very easy to give a game top marks if you love it that much. Okami isn't perfect, but no game is and despite that it still deserves top marks considering it is a games on an "old" platform.
This is as close to a Zelda game on the PS2 as you could find (even more so than 'Beyond good and evil'). And that's not a criticism, the quality essence of Zelda is something game makers should strive for. From the slighty annoying (but nonetheless entertaining) tiny companion to the quasi-RPG character development this really hits the nail on the head.
Personally, I don't care for the graphical style. A lot is made of this game's original presentation, and while unique does little to enhance the game itself. It may not have the splendour of a 'Final Fantasy XII' or 'God of War' but by it's own standards is almost perfect.
Of course, graphics do not make a game, and it is playability where Okami shines. An almost perfect learning curve and engaging story with a witty script kept me hooked right until the end. The 'Celestial Brush' aspect is clever and never feels like it disrupts the flow of the gameplay, even during the epic boss battles (the fights with giant spiders are particularly inspired). On occasion I found myself accidently using one technique when I intended to use another but this was the exception rather than the rule and the controls are remarkably responsive.
The sound can become repetitive at times. There is no speech, every character speaks in a kind of made-up babble and I found myself hurrying through conversations but this does little to detract from game; it will never allow you to miss something important.
Overall, this game (considering it has been released towards the end of the PS2s life) really shows the full capabilities of the system and could easily have been a XBox360 or PS3 release.
It is not the kind of game that will last for 100+ hours but neither is it the kind of game that you can fit a quick half hour in.
Okami is entertaining, challenging and a joy to play.