The first thing I have to say is this: due to the sheer volume of the game, a purchase is pretty much necessary to enjoy it in its entirety. If you rent it you will only scratch the surface of what this phenominal platformer has to offer.
Graphics. It's not aiming for realism, but the rich, vibrant colors and well-defined visuals are on par with Blu-Ray animated movies.
Level design: The design team must have consisted of at least six geniuses and three madmen. Every galaxy has it's own unique hook while still retaining the core gameplay elements. Examples: the Hightail Falls galaxy has Yoshi (in his best appearance ever, period) eating peppers that allow him to run at blazing speeds up vertical surfaces, while the Supermassive galaxy is, well.... self explanatory. Even if you die repeatedly, the game's too fun to frustrate.
Gameplay: Everything feels natural within about thirteen seconds. New control elements work as well as you possibly want, with never a cheap death.
Story: It makes very, very little difference in a Mario game, and you can probably guess it anyway.
Replay Value: Extremely robust. You can beat Bowser with about seventy stars, but you'll be compelled to obsessively hunt down all 240.
The advertisements didn't do it justice, so I'll help: BUY IT!!! BUY IT!!!
You get exactly what you want with this game, and then worlds more. It earned a 10 because of that.
Mario Galaxy 2 brings a lot of new ideas to the table for Mario. Especially in the way of new powerups. But as I kept playing this game it just didn't feel any different to me then the previous game in the series. It's fun, for sure. But it just feels like we've been there before.