As I mentioned on the Speed Metal page, the distinction between Thrash (wikipedia) and Speed is slight (or non-existent, depending on your perspective).  After listening to both sets of samples, though, I do think there's a difference.  I'm not really sure how to qualify it, other than to say that Thrash Metal tends to have a bit more ominous of a sound.  I also think the emphasis is more on the composition as a whole, rather than just on playing as fast as possible, or on showcasing the virtuosity of the guitarist.  This isn't to say that Thrash is any less technically demanding or impressive than Speed; it just seems to me to be a more evolved subgenre.  I also like it more than Speed Metal...again, it's difficult for me to put into words exactly why, but I think I like the more aggressive edge of Thrash.