The "Metallica for Prog Related" Poll

Since Ivan mentioned the AllMusic Guide, here is their definition of Thrash music. 

Progressive metal is a sub-genre of heavy metal music which blends the powerful, guitar-driven sound of metal with the complex compositional structures, odd time signatures, and intricate instrumental playing of progressive rock. Some progressive metal bands are also influenced by jazz fusion and classical music. Like progressive rock songs, progressive metal songs are usually much longer than standard metal songs, and they are often thematically linked in concept albums. As a result, progressive metal is rarely heard on mainstream radio and video programs.

HISTORY

The origins of progressive metal can be traced back to progressive rock bands from the late 1960s and early to mid-1970s such as Yes, Pink Floyd, Emerson, Lake and Palmer, Jethro Tull, King Crimson, Genesis, Gentle Giant, Focus, early Queen, Kansas, Atomic Rooster, Uriah Heep and Rush. The latter 5 also often blended metal elements into their music. However, progressive metal did not develop into a genre of its own until the mid-1980s. Bands such as Rainbow had many qualities of progressive metal. Bands such as Fates Warning, Queensrÿche and Dream Theater took elements of these progressive rock groups – primarily the instrumentation and compositional structure of songs – and merged them with heavy metal styles associated with early Metallica and Megadeth. The result could be described as a progressive rock mentality with heavy metal sounds.

Progressive metal received mainstream exposure in the early 1990s when Queensrÿche's "Silent Lucidity" (from 1990's Empire) became a radio and MTV hit. It was not a typical progressive metal song, but its popularity increased the profile of other progressive metal bands. In 1993, Dream Theater's "Pull Me Under" (from 1992's Images and Words) became popular on radio and MTV. In the 1990s, bands such as Pain of Salvation, Opeth, Ayreon, and Symphony X developed their own signature sounds.

Pain of Salvation drew heavily on more obscure 1970s prog acts. Ayreon stayed with the traditional Prog Metal themes, but mixed them with rock opera influences. Symphony X married progressive elements to power metal, bridging the gap between the two sub-genres. Steve Vai's former singer and heavy metal band Strapping Young Lad's singer and guitarist Devin Townsend combined elements of post metal and ambient with traditional progressive metal on his first two solo albums Ocean Machine: Biomech and Infinity. Opeth combined their prog influence with death metal. Another influence on prog metal was "technical metal" bands, such as Meshuggah, Watchtower, Atheist and Cynic, which played complex song structures and used virtuoso instrumental playing.

Bands like Sun Caged, Dominici and Circus Maximus are influenced by traditional progressive metal and several of the first wave of 1990s bands. Bands such as Dark Suns, Disillusion, or Conscience are influenced by emotional progressive metal bands like Opeth, Pain of Salvation, Green Carnation and Anathema. Sweden's Tiamat have also been influential in the progressive metal genre, especially on their 1994's breakthrough-album Wildhoney.