Re: Why is the guitar "nut" a big deal?
The nut affects the tone and resonance of the guitar. A good nut will improve resonance and slightly colour the tone. A bad nut will resonate poorly and muffle the sound of the strings.
Different nut materials resonate differently, and there is really no "perfect" nut material. Graphite has a very even response and resonates very well, which is why it is popular as an after-market mod. Bone resonates well too, and sounds a bit less mellow than graphite. Brass has excellent resonance, but tends to sound very bright. Corian is a synthetic material that is very similar to bone. There are other nut materials, but those are the most common.
A badly cut nut, regardless of type, can cause poor intonation, and may also cause strings to break.
Hope that helps you understand the importance of the nut.