Things that are really difficult aren't ones where you have single, hard problems, but where you face a mix of a multitude of trivial, tiny problems.
Simple thing: You want to connect to PC. So you need cross-over. Now, where? Do you use just an expander cable, with 1:1 pins? Or a null-modem one? Now if it's not crossed, the good place to cross the lines is on the board. But where? Between the MCU and the transceiver or between transceiver and the cable? And if you cross them, which pins do you attach them to? Connecting two output ports together, just like connecting two input ports together, won't do much good. And it all depends where the cables are crossed. And now consider you want to route the control lines through the same MAX232-pin-compatibile. Routing them manually, on a single-sided board. (we're not talking about pro experts who know this all, we are talking about newbies!)Do you happen to remember pinout of MAX232? While one pair is reasonably laid out, the other is one of worst, sickening examples of pinout design. When you make your first MAX232 based interface for the first time, you are thoroughly lost. IMO this is very valuable idea.

I'd suggest some more "standards" the same way. External memory, memory mapped IO, binding external code&program memory, attaching external oscillator instead of a crystal, driving high-power, high-voltage devices, buffering IO data/analog signals, leaving a handy hook for reprogramming, interfacing buses like CAN, SPI etc, talking to different voltage level devices (3/5V), providing enough of good, stable power, extensions for reprogramming eeprom in circuit, and probably quite a few more.