That's odd, I used to be able to play on High quality in World of Warcraft with my 9400M, but now I can't. Maybe they improved the graphics in Cataclysm.
Anyway, there are many graphics settings like shadows, reflections and particle density. Some use more processing power, yet do not look that much better. After playing around, I found that Shadow Quality and Liquid Detail cause the biggest hits in performance. Filtering did not, surprisingly. It's reasonable to assume different graphics cards will perform differently, depending on how they're optimised.