Originally Posted by jackhammer

The time and origin are a big part of it but that is mainly because of the influence of Punk in that Metal generally got harder and faster from 79 onwards so that awarded itself the tag of NWOBHM in that the new bands at the time stepped away from Blues based music and became more streamlined and economical. Not always of course as bands like Angel Witch incorporated much more melody and breakdowns than their counterparts but on a general scale Metal in the U.K became more lean and less bloated.

Thrash Metal definitely took many influences from Punk and the emerging Hardcore scene but it also too influences from NWOBHM and without those sounds the Metal scene would look completely different and that is why NWOBHM is still heralded as one of the most important scenes in metal.

Just as the Punk scene gained a few bands that weren't strictly Punk at the time and just happened to be playing at the time and so got lumped in with the genre the same happened with Metal at this time but the ethos was the same. Play harder, dirtier and faster (when appropriate).

The best band to demonstrate this was Venom. They were amateurish, out of tune and lacked a cohesive sound but the attitude and raw sound is the perfect stop gap between these genres.