ClojureScript is probably not the best language to use if you want few layers of abstraction. I personally love it, though.
The best way to think about this development is that it is a client-side compiler for ClojureScript. In client-side development, you are constrained by the limitation that all code must be in JavaScript to be executed by the Web browser, which is why this compiler "compiles to" JavaScript instead of machine code. JavaScript is the assembly of the Web, right now (until things like WebAssembly get more useful and popular). Once the ClojureScript is compiled to JavaScript, the JS can be executed by the browser (which of course involves an additional JIT compilation step). This might sound "dirty" if you are used to thinking about non-Javascript development, because it involves two separate compilation steps, but it is unavoidable, once again until something like WebAssembly becomes popular.
So, this is a client-side compiler for ClojureScript. A server-side compiler already exists, which is what you would use if you wanted Emacs/Slime support, or if you were developing a "real" Web application for deployment, because in that case you would want to pre-compile to JavaScript on the server, and distribute the compiled JS to the client.
It doesn't really make sense to talk about Emacs/Slime running on the client, until someone makes Web app for it. You could use a socket-based REPL to communicate from the client to the server but I don't really see the benefit, when you could just use the server-side compilation model and avoid the network overhead.
The best way to think about this development is that it is a client-side compiler for ClojureScript. In client-side development, you are constrained by the limitation that all code must be in JavaScript to be executed by the Web browser, which is why this compiler "compiles to" JavaScript instead of machine code. JavaScript is the assembly of the Web, right now (until things like WebAssembly get more useful and popular). Once the ClojureScript is compiled to JavaScript, the JS can be executed by the browser (which of course involves an additional JIT compilation step). This might sound "dirty" if you are used to thinking about non-Javascript development, because it involves two separate compilation steps, but it is unavoidable, once again until something like WebAssembly becomes popular.
So, this is a client-side compiler for ClojureScript. A server-side compiler already exists, which is what you would use if you wanted Emacs/Slime support, or if you were developing a "real" Web application for deployment, because in that case you would want to pre-compile to JavaScript on the server, and distribute the compiled JS to the client.
It doesn't really make sense to talk about Emacs/Slime running on the client, until someone makes Web app for it. You could use a socket-based REPL to communicate from the client to the server but I don't really see the benefit, when you could just use the server-side compilation model and avoid the network overhead.