First, the obvious. Thrash metal is fast. It doesn't necessarily have to be 200bmp with palm muted 16th notes and blastbeats, but it has to be at least on the upper side of midtempo. This is shared with speed metal, though speed is less integral to thrash.
The thrash metal rhythm section generally abides by an 8th or 16th note skeleton, with different variations to create a more "jerky" sound. Triplets are employed extensively as well, and gallop style rhythms are also used. Examples include "Battery" by Metallica or "Impact is Imminent" by Exodus.
Thrash metal is less melodic than speed metal, as well. Guitar melodies are few and far between, instead, the riffs sound like choppy patterns of notes. Evil sounding intervals and chords are used, and the vocals are harsh. Again we have e to differentiate from speed metal. Speed metal is generally as fast and often faster than thrash, but it tends to have a very steady, laid back sound in comparison. Listen to "Alison Hell" by Annihilator. That is not thrash, it's speed metal. The same goes for "Overkill" by Motörhead and "Exciter" by Judas Priest. Then compare that to almost anything by Megadeth, who use complex riffs, sudden fills, rundowns, groove sections, and especially tempo changes.