OK, then what are the biggest, baddest offenders of all lossy
compression techniques?

I think it'll be hard to find such a generality. What destroy's most
depends on the encoder you use.

Some MP3 encoders cut all ferquencies above a hard coded value, the worst
I've seen was cutting everything above 15000Hz, wich of course can destroy
a lot of music. So for that decoder the cutting of frequencies was possible
the worst offender.

Different encoders also use different psycho-acoustic models. Some encoders
use rather bad models wich in those encoders can be the worst offenders.

I doubt you'll find one single mechanism wich is the worst offender in all
encoders.

Of course the bitrate is allways important, but it's not an offender. The
whole purpose of lossy compression is to make a compromize between data
size and quality. When someone encode at a low bit rate said someone does
so because the size is a lot more important than the quality. When someone
purposefully encodes to low quality the resulting low quality shouldn't
offend said someone.

/Jonas