Not implemented:

- Input. It would be straightforward to provide a fixed-size input
  buffer and read from that. It would also not be difficult to load the
  input into shared memory, but that would be overkill.

- Any sort of interactivity. The program runs in a tight loop until it
  completes, so examples like Conway's life are not usable.

- Interactive I/O (sensing a pattern?). This needs to be a blocking
  function call to wait for input, which isn't easy in JavaScript.

- Debugging of any sort. Use the web browser's tools.

Yes there's a useless exports section, but it may be helpful to read the
contents of main memory.

I am suspicious that the authors of WebAssembly leaned heavily on
Brainfuck for inspiration.