I disagree that Numbers are relatively uninteresting. In fact, I think any language designer would tell you that numerics are one of the hardest areas to get right. It's also the area where Clojure is weakest in terms of interop, I'd say. ClojureScript, for example, only has host Numbers (ie double-precision floating-point)

The main thing that Clojure does in terms of being designed for interop is to require the use of host values/objects. There are no wrapper/proxy/delegate/whatever objects. This has big implications. It (unfortunately) means that there is behavior you simply can't have in core, such as reflection or continuations. That's a big part of being interop friendly: Intentionally specifying fewer things. If you have very strict rules for how something like strings behave, then you might wind up in a situation where you need to create a ClojureString class or something like that.

Additionally, the '. (dot) special form is like a giant "INTEROP HERE" sign, which is extremely useful when porting code from CLJ to CLJS. You can literally grep your source for /\./ and find platform specific code.