Keeps bothering you to install the "helper" application for basic functionality. Puts arbitrary limits on how many videos you can download within a certain period, this limit can be bypassed by uninstalling the extension, then reinstalling.

I have no problem with paying for open source software, but the fact that payment is required for some functionality should be made clear on the extension's page, and before the user spends potentially hours waiting on a video to download, only to find out this extension superimposed a massive QR code over the downloaded video.