Simon's BASIC was buggy. Some worked great some didn't work well at all (like line renumber) IIRC there was an updated version out there..
Second, I think the power of the 64 was its' limited BASIC but it was a good start (still the best line editor of most systems, just cursor up change the line, press return and done), The no-frills in the language but not in the capabilities made it a platform where there was no one definate way to do graphics or sound so there was a lot of experimentation most good, and some downright awesome. I think not having those features baked in the ROMs made the community a lot more dynamic.
> I think the power of the 64 was its' limited BASIC
I am torn about this. Forcing people to use machine language for anything serious makes sense. The 10,000-strong C64 software library exists. Your point about the great screen editor is well taken.
On the other hand, what about all those who would have experimented on their own more if they could have done things with CBM BASIC 2.0 that Atari BASIC has out of the box? Applesoft BASIC has more graphics support than CBM BASIC, too, and there was never a problem with converting experimenters into "real" developers.