- hiccup syntax / sexps are trees analogous to the HTML you're constantly generating, and first-class supported data structures supported by the language which need no pre-compilation/transformation step, unlike JSX
- syntax is a good 20-30% less verbose than JS (yes, even ES6), especially when the core lib is taken into account (things like partition, zipmap, threading macros)
- one extremely battle-tested dependency resolution story, not named NPM - it just works
- incredible development tooling innovation, best of class for frontend web development
- the sequence abstraction and dozens of functions that operate on it, rather than Object.assign, Array.map, etc
- best-of-breed dead code elimination via Google Closure Compiler, meaning much of third party libraries and development-specific code ends up weighing less / nothing in production builds
- sets and set operations built in, along with tree walking, diffing and string utility libraries, included with the distribution
- no more defensive copying, thanks to immutability everywhere