I Am no insane music theorist, but when determining a key, it's either best to set down a key to use int he first place when writing a song or just go by whichever relative basic major minor scale sounds good over the song. Esepcially if you are into metal, look into using Phrygian mode and such for a wicked evil minor sound, or just standard minor can be good. It is all up to you. As far as determining it in a chord progression most metal uses a base note or the guitars tuning as the key for the song. Though if you are into Megadeth then you would find this to be untrue. Though looking at any of Metallicas early stuff the whole first album is pretty much in Eminor. Since they spend lots of time palm muting open E notes all the time.
As for your origianl question about solos and leads, I say think in your head as you listen to the progression what cna i do that would be interesting or cool or musically worth while to play. If all you are doing is wanking with simple 3 note triplet patterns at excessive speed then you are limiting yourself to simple melodic scale runs. The trick i have found is to take the notes in a scale and run them up and down the fret board up and down octaves. I know not any special music theory term for this practice but i have noticed it in many classical pieces, neo-classical guitar solos, and metal as well. The trick here is to make the notes help to add to the feel of the song.
Another thing you could try is mixing up the patterns you play with 3 note at a time. Doing different triplet patterns across the fret board using whatever scale you decide upon. Move on two strings, 3 strings, 4 strings, whatever you can manage and make sound good. I've found some cool stuff can be accomplished using a whole tone scale on the top three strings. Also try using Diminished licks and runs. Augmented notes and such. I know there are some theory rules for when this is ok and when it is not musically viable. Under that term musically viable i mean pleasant to the ear. Someone else can fill in the blank on that one.
Yea, in other words take what you know and either build off it in creative ways through thinking and experimenting with whatever crazy guitar playing ideas you get or live with lame wanking with simple melodic 3 note patterns. Look to your influences for ideas to build off of and perhpas create some of your own as a result of combinding a few or just making way for your own ideas. I am a metal guitarist and it took me all of a year to nail down decent shred lead and contol after 3 years of rhythm playing. If you want some random stuff to work with just contact me or whatever, i have more than enough time to tab out some random ideas.
Hope that helps dude,
graphics_ninja@yahoo.com
[Edited by Seiko_Hejiro on 01-04-2003 at 02:51 AM]