Size and Weight:
When its fossils were first discovered in Mongolia's Gobi Desert, in 1946, paleontologists debated whether Tarbosaurus was a new species of Tyrannosaurus, rather than deserving its own genus. Clearly, these two carnivores had a lot in common--they were both huge meat-eaters with numerous sharp teeth and tiny, almost vestigial arms--but they also inhabited opposite sides of the globe, Tyrannosaurus Rex in North America and Tarbosaurus in Asia.
Lately, the bulk of the evidence points to Tarbosaurus as belonging to its own genus. This tyrannosaur had a unique jaw structure and even smaller forelimbs than T. Rex; more important, no Tarbosaurus fossils have been found outside Asia. It's even possible that Tarbosaurus had evolutionary precedence, and spawned Tyrannosaurus Rex when some hardy individuals crossed the Siberian land bridge into North America. (By the way, the closest Asian relative of Tarbosaurus was an even more obscure tyrannosaur, Alioramus.)
Recently, an analysis of a Parasaurolophus fossil revealed numerous Tarbosaurus bite marks, in patterns indicating that this tyrannosaur methodically scavenged its victim's already-dead corpse rather than chasing it down and killing it. This doesn't conclusively settle the debate about whether tyrannosaurs were hunters or scavengers (they probably pursued both strategies, as necessary), but it's still a piece of valuable evidence.