This is a very very early project but mostly I think it'll be interesting to see how well they manage to utilize type annotations and how well the Clojure ecosystem provides this? (ie, is general Clojure code mostly untyped or is there enough for compilers to deduce most actual types?)
Why would this be important? Well because they are relying on LLVM for optimization, and while that's good if you know type info (via annotations or lower tiered profiling), there's a lot of code to start using this and those optimizations can be fragile.
If you read the articles posted here in the past year on the Erlang JIT they mirrored this sentiment, earlier "advanced" JIT attempts built on classic static language techniques like LLVM prefers failed because they didn't have enough type information, whilst the "simple" JIT that mainly removed overheads and concerned more with local types was showing good promise without too much engineering headache.