C64 was an excellent tool for learning low level bit twiddling
The lack of a powerful BASIC interpreter directed my attention to learning 6502 assembler so I could start writing 3D wire frame vector graphics. I learned how to write cycle accurate timing code to drive the VIC II to make side borders disappear and with raster interrupts to make the the top and bottom borders disappear too. I also wedged in my own BASIC tokenizer and interpreter to extend the BASIC to provide better structured programming (while/wend, procedures, repeat/until) and sound, graphics and disk support - all this taught me how to structure large projects in assembler and how to write compact and efficient code.
I spent hours pouring over the disassembled C64 BASIC and Kernal ROMs and learned the art of reverse engineering from the object code. I figured out the tape format, analyzed the read/write characteristics of the tape drive head and re-wrote my own tape turbo loaders.
With the aid of an annotated ROM disassembly of the 1541 floppy drive I figured out how to write disk turbos and I hacked up my own fast formatting tools and my own file system.
By the time I was 17 I had acquired the the Super C Compiler and I learned how to write C on a system that had a 15 minute edit-compile-link-run turnaround cycle(!).
All this 1MHz 8 bit goodness taught me valuable lessons in programming efficient code and the trade-off between compact code and fast code. I learned how to twiddle hardware, bit bang data down wires and push a system to squeeze a little more performance out of it.
I was fortunate to have the time and energy and the right hardware available in my formative years, so I am grateful for Commodore for producing the quirky and hackable C64.