What is...

Playing fast for the sake of it is silly. Some songs are fast, some songs are slow. Hearing a thrash album with the same drum beat on every single song is torture to me. But that's kinnnnd of what it is. Some specific genres stick to to certain tempos. Dance music is a good one for noticing that cos each sub genre has it's own tempo, but that's the point there. There's no point in bands being fast for the sake of it in my eyes unless it's exactly what the genre is, like grindcore... But come to think of it I know slow grindcore songs.
 
Personally speaking, I love speed metal in general, so speed is generally welcome.

Other than that among my 15 dearest albums ever is Candlemass' Epicus Doomicus Metallicus.

I really don't think a thoughtful (ie normal) music listener equates speed with quality. Having said that, I do feel a bit empty inside if a thrash album doesn't have a speedy number! :p
 
I like fast albums... what's wrong with that? Slow music in general bores me. There are occasional doom songs that I like, and occasional groove bands that I like, but in general I like fast music! You want to fight about it? :D I also don't know if it's coincidence but when the 80s bands that I like got obsessed with slowing down and grooving all their albums turned to shit (for me).

Having said all that, there is such a thing as too fast...
 
Speed's always fun, but in thrash (I assume), which was founded a lot on speed and aggression, a lot of bands will have to compensate for if they aren't going for the (generic fast riff here) and the (wild sonic speed solo here) thing. Also I always believed this slogan: "If it ain't heavy, it ain't metal!" although I could see how it doesn't really fit.
 
It depends on the song and on the mood I think, but I prefer songs with faster parts and slow parts and more harmony changes.
If in a song always the same the same riff gets repeated it's boring (like many modern thrash bands also do)
The faster parts need the slower parts I think aswell cause when it's always fast you can't define fast anymore.. just my opinion.