Upon discovery, Dilong was the oldest known unquestionable tyrannosauroid. It was one of the most complete early "tyrant lizards" too, and one with a plethora of unusual features, the most newsworthy of which was the first direct fossil evidence that tyrannosauroids were feathered. They weren't like the feathers you would find on modern birds, but bristly fibres or "proto-feathers", and they covered its entire body.
Typical of early tyrannosauroids — the coelurosaurian dinosaur lineage that culminated in the Late Cretaceous tyrannosaurid, Tyrannosaurus rex — Dilong is small and gracile with a long and shallow snout and relatively long arms with three-fingered hands. Unlike the larger and more advanced tyrannosaurids, its bones are less aero-textured, and its pelvic bone is less robust but ends with an enormous bony boot. The hindlimb proportions suggest Dilong was a decent runner, but it wasn't as fast as similar-sized tyrannosauroids because it had shorter lower legs.
In a 2014 study, Dilong paradoxus, along with Proceratosaurus bradleyi, Kileskus aristotocus, Guanlong wucaii, Sinotyrannus kazuoensis, Juratyrant langhami and Stokesosaurus clevelandi, was found to be a proceratosaurid tyrannosauroid.
The species epithet, paradoxus, is derived from the Ancient Greek "paradoxos" (contrary to expectation) and refers to its unusual features.