I know that having your nails too long or short is bad, particularly for classical/fingerstyle guitarists,
but what constitutes to having 'good' nails?
Bonus points for pictures/external articles
This video has some good advice on shape and how this affects the sound, and also how to shape it. It is from a classic guitar point of view, where it is important to have a broad contact with the string to create a high quality sound.
Additionally this article has some good theories about what to consider when forming your nails, and emphasize the different needs for different classical playing styles:
This second video is more aimed at electric guitar which is more of a practical approach for supporting different playing techniques, where you may not have the same length as for classical.
I have some perhaps-unorthodox ideas that I've formed over the years. I started with pointy tips and they'd always break if played too aggressively. So I (foolishly) tried putting lots of polish on them to keep them from breaking (and look cool).
But when I started concentrating on tremolo technique and real demands for speed and articulation, pointy tips is a drag (and polish is stupid). You want just a bit of nail, with a soft curve. If you practice the Apoyando (Rest Stroke), where you use both the flesh and the nail, and where your finger comes to rest against the next string, you will naturally discover the proper length. Too long and you can't touch the strings easily with the flesh of your finger tips. This is bad for speed because your fingers have more difficulty finding the strings. Too short, and your tone won't have the desired attack.
The way to look at the nails to examine their length is not from the top (the part that you polish), but from the bottom. Turn the palm of your hand toward you and look at how much nail is peeking-out past the tip. This is the way the strings will see it.