So what instruments are hard to encode *usually*? I know people have talked about Harpsichord, but is there any other hard-to-encode things? And what instruments are easy to encode?

Rare artificial signals for all encoders. For MP3 ; precussion, keys - pre echo. Ringing, dropouts with quite music - vocals, guitars.

In classical, I find strings are always a tricky area. They're usually where I notice a poor encode first.

Cymbals always highlight the failings of less-than-great MP3 encodes, they sound artificially jangly on the very high end.

The lead singer from the Stereophonics, Kelly Jones, has an amazingly gruff, husky voice which used to sound AWFUL when I first encoded one of their albums - this was a good eight or nine years ago AT LEAST, I was using the CannaMP3 Encoder (oneclick solution) with, I think, the Xing MPEG encoder! Of course, Lame handles it just fine now, and has done for several years - but if you've got a low-bitrate encode, his voice is an excellent indicator of whether the quality is going to be good or not, regardless of the format. (listen to "Bartender And The Thief" from their album 'Performance And Cocktails').