The thing i like most is the easy compiler / text editor built in to the web page itself.
And ... the utter simplicity of getting a graphics window working. Going back to, the 1980s, it has always been a stretch to get graphics going in a programming language. From the TI Basic on the TI-994a, which i never really understood... to the IBM PC Basic which simply drew lines over your text as you typed your BASIC in...
to Borland Turbo Pascal, which provided 'standard' graphics modes, but not the 320x200x256color that every cool game used. For that, you had to drop to system calls to set up a special call to a software interrupt in MSDOS.
In Elm.... at least in this one case... they have made simple things simple.