OK, I don't really get the difference between remasters and remixes, but are there any that are essential? It seems in a lot of cases, people like the feel of the original despite arguably crappy sound.
Also, are there any albums that could really use a good remaster? Just heard Ace of Spades and was thinking it doesn't sound particularly great, but I'm sure many world argue with me on that one.

Remasters use the same mix, but normally just boost the levels and compress the dynamics in most cases.

In a remix you can clean up every track (as in 'layer') separately and give individual instruments or elements more prominence than before.

I'd love to hear a good remix of BÖC's "Imaginos". The original production is very complex, layers upon layers of stuff, but it's very uneven and muddy.

… And Justice for All is a great example of an album that should be remixed so that Jason’s bass can be heard next to the other instruments. The album has been remastered which changes the sound of all instruments together, but did not bring out the low end.

I don't really think the problem with And justice was the bass. It just sounds so sterile and mechanical. No amount of bass is going to fix that.