Webcams that don't support 640x480 require fiddling
If you attach a webcam that doesn't support 640x480, because all the import options have this resolution set, it doesn't work. The capture preview window remains black.
With my old Sony camcorder attached via USB, only 320x240 is supported:
tim@Equium:~$ v4l2-ctl --list-formats
Index : 0
Type : Video Capture
Pixel Format: 'MJPG' (compressed)
Name : MJPEG
tim@Equium:~$ v4l2-ctl --list-
Size: Discrete 320x240
Of course, it's pretty easy to fix once you know all this, but it's not exactly noob-friendly.