There is a very minor difference between the "/dev/console" terminal port and any other "/dev/ttyxx" terminal ports: whether the port is optional.
For the console port, the port is on a card that came with the base PDP-11 system (not an optional add-on). On some models of PDP-11 the port was on a separate card (or set of cards, e.g. the KL11 on a PDP-11/20), while on other models the port was actually included on the CPU card itself (e.g. the PDP-11/23). When the console port wasn't a KL11, it was something that was KL11-compatible (so the operating system could pretend it was a KL11 and it would work no matter what model of PDP-11 it actually was).
The tty ports were typically for ports that were added to the system (either as factory-ordered options, or options added later). These were typically added using options such as the DL11, DJ11, DH11, or DZ11.
From a software perspective, the KL11 and DL11 were essentially the same thing, but the KL11 came with the system (often in the CPU cabinet itself) while the DL11 was an option. The KL11 and DL11 each supported a single terminal, but options like the DJ11, DH11 and DZ11 could support up to 16 terminals (8 for DZ11).
For Unix, there was nothing special about the console port except that its driver was always included in the kernel (because every system had it). To support other terminal boards (DJ11, DH11, DZ11, etc.) you might need to reconfigure the system with additional drivers. In particular, on Unix v7, if you install the system from tape the 'stock' kernel will only include support for the console KL11. If you want to use additional terminals you need to reconfigure the system to add support for DL11/DJ11/DH11/DZ11/etc. options. For more details, take a look at Setting Up Unix - Seventh Edition in the "Reconfiguration" section.