metal more often than not has a darker lyrical message, and a structure all its own. its not based on chords in repeated, semi-simplistic fashions. (exceptions to this are found a lot in 90's grunge music actually) in metal, the technical skills of the musicians are on display in most genres, but the critical difference is atmosphere. Metal music, especially the darker metal genres have an atmosphere all their own. usually dark, desolate, sometimes made possible by purposefully poor production. even when the production is good, the downtuned guitars, crunchy riffs and low-end distortion creates a similar aura of doom. punk is fast, but punk is also sloppy. punk shares a lot in common with thrash metal in the department of speed, but thrash artists employ more guitar theatrics as well as lyrics usually based on hell, politics or the apathy of life.
hell i could go on and on but fortunately for you fuckers, i have an exam to go fail.