I love how you could mix 6502 assembly and also create macros.

For example, you could declare something like this to move two bytes from F% to T%:

  870 DEF FNMOVW(F%,T%)
  880 [
  890 OPT pass%
  900 LDA F%
  910 STA T%
  920 LDA F%+&1
  930 STA T%+&1
  940 ]
  950 =pass%

And then reference it from somewhere else like so:

  220 OPT FNMOVW(&52,&4B)

And the resulting code would be inlined into your assembly. It made for shorter and more readable assembly.