There are two contrasting art styles in Mirror's Edge. First, the story narrative is told through hand drawn cutscenes that resemble a certain auto insurance agency's commercials. These vignettes are pretty short and merely set up the story in between chapters, but there are animated really well and have a clean aesthetic.
The other art style that will impress most people is the actual gameplay. Sprawling 3D environments are presented with a dystopian sterility, meaning that Mirror's Edge uses a very limited color palette. All of the locations are impossibly clean and the visual style is saturated to the point that everything feels like it could be a mirror. Since it is mostly cool colors, like blues and greens, when "Runner Vision" is activated, it makes a huge difference. "Runner Vision" is basically a path-finding tool in Mirror's Edge. Key objects will be highlighted in bright red so that you can find your way around the level. It may be a little hard to accept at first, but the simple style is important so that you can focus on the core gameplay.
The audio and sound design is also top notch. Each character's voice actor is convincing and appropriate for the character design. The main character, Faith, breathes heavily as she runs and every footstep sounds different and paying attention to little auditory clues like that is important to timing certain jumps. This is the type of experience that needs 5.1 surround sound or 5.1 headphones. It is fine if you don't have those, but it will enhance the entire package so much more.