Can someone explain to me what is the problem of using Director 11 on a PPC
Mac? Why the product page says an Intel Mac is required? I downloaded the trial
on my PowerBook G4 and didn't find any problem so far.

It is not "required" that you use an Intel Mac. It is just that Adobe
does not support its use on PPC Macs. I have heard that it works fairly
well on PPC with very slight glitches that don't often interfere with
the use of the program. But if you use it, just know that you are using
it in an unsupported fashion (at your own risk), and any problems you
encounter will not be fixed by Adobe.

>... But if you use it, just know that you are using
> it in an unsupported fashion (at your own risk), and any problems you
> encounter will not be fixed by Adobe.

This covers all use of D11 actually....