I are new here, so don't be mad if I posted up something you guys knew already or what-not. I'm just here to tell you guys what I think makes the guitar aspect of a song successful...
1. You have to play it like you're telling a story. Your guitar has to flow, it can't just jump from one part to another or it'll sound like mixed up pieces, not a whole thing. Like a story, you have to let it flow well, from angry, to sad, to happy.
2. You don't have to play 100mph to get a good solo. Sometimes less is more. You have to make each note count! One well placed note can go a looong way. Toss in a little bit of fast playing here and there to add a little bit, but don't overdo it.
3. When playing lead, you can't just be "scribbling" with your guitar... Paint with it! Use dynamics to build effect. Even a simple chord progression can sound beautiful if you use dynamics properly! Use your guitar to a paint a picture with sound, lure in people with simple, but catchy, hooks.
So this is what I think makes a good guitar part in a song. I really dislike alot of rock and metal music because the guitarists of certain bands just play fast. I mean, yeah dude, it's cool, but after a while it starts to sound stupid. A guitar should be able to move you the same as lyrics can. It should be able to speak to the listener, not attack them with sound. Well, this is all I have to offer for now.
The Guitarists Guild of Gaia
The Original Guitar and Bass Guild