Is it the instrumental, the lyrics, or a combination of both?
Where do you draw the line between Metal and Rock?

Metal = Judas Priest
Rock = AC/DC

I've come to realize that the only way to get a handle on what makes a given genre what it is is to listen to it. Over, and over, and over again.

Metal is just like any other concrete genre of music in that it has a boundary that encompasses music and divides it into its own little subset. It's definitely a guitar-driven form of music, and it's just something you grow to recognize after a while. There's a certain sound to a metal riff that sounds different than say a rock or industrial riff, and the songwriting can go in a number of directions.

The line between Metal and Rock is often a bit blurred in the more traditional genres, and I'd say the line is crossed when the music begins to get more aggressive and dark and heavy - rock bands are usually more simplistic with less focus on the riffs, from what I've observed.

Metal: The most misunderstood genre of music ever. Which is why I love it so much.

I never cared about popularity, how people interpret it etc. I just think it is the BEST, or the WORST(it's true ), music on earth. I have taken so much shit for being a metalhead, but I just feel sorry for those who can't appreciate the might of metal

for me it is just the art of heavy riff, asserved to a number of different purposes...

Seeing as how metal is a type of rock I don't really bother to draw a disticnt line between the two.

Metal is something that the overall feeling and attitude of a song gives off. A metal riff could be in blues scale, pentatonic scale, modus, or just random notes and sound metal, it's something you can feel.

that's right
the problem is that it would sound a bit much vague in an encyclopedic context

I second this.

It can be a lot of things. Part of it is evolutionary, did the sound evolve from metal roots? Part of it is instrumentation. Part of it is a certain darkness, or heaviness. Part of it is band image. Ultimately, it's a lot like Pornography. You know it when you see it, unless you're 14 and hang out at Hot Topic; then it's everything.

I third this. Given that I think of AC/DC as the defining rock band and Judas Priest as the defining heavy metal band, that'll do for a definition.

Would this line have started with Sad Wings? Or something like Hell Bent For Leather... or Defenders of the Faith?

Metal is too large to be encompassed into one single definition that covers it all because of how deviant some sub generes are, even some individual bands.

The most generic I can get is: riff centered, with drums to make it 'go forward'. Ya, that's pretty damn broad and poor.