I found it very convenient, that the initial config will use default values after the countdown runs out. This helps if you don't have a rs232 cable. It would be great, when before the countdown, there would shown, what the defaults would be chosen when the countdown finishes.
The homepage (opnsense.org): The buttons are jumping around. While this is a nice visual effect, it makes a bad user experience. I find myself constantly "hunting" the buttons.
WebConfigurator: The design of the web configurator wastes lots of vertical spacing. Nowadays, Laptop-screens tend to be very low. On my laptop (1360x768) the "interface status" of two interfaces takes three pages to scroll down. Judging from the font size, it all would fit onto one screen.
I find it somewhat strange that "Reboot" and "shutdown" are located in the "diagnostics" menu. I constantly find myself searching them in the "system" menu