I was wondering what the security implications are of using non-free drivers on a linux laptop. Especially for graphics and wifi.
Drivers for hardware devices in your system typically operate with a high degree of privilege. They need to, in order to interface with your hardware.
So the question is - how much do you trust the code you're running at a high privilege level? For that, you'd probably have to consider source of driver, whether it's signed by the issuer, can you inspect the source code etc.
Of course, just because you can inspect source code, doesn't mean you'll spot a problem. See also: Shellshock, Heartbleed.
There's a risk to running any code on your system, but usually a reputable vendor has positive incentive to ensure there aren't any particularly egregious vulnerabilities.
You may, however, find that there's also incentive to pretend weakpoints don't exist, or otherwise be lazy about releasing updates.