I'll be the first to ask: whats wrong with JSON?
(If you don't know why this might matter, try opening your browser's Javascript console and evaluating 10000000000000001)
That's my peeve, though. I suspect that Tom is probably more concerned with readability. TOML also looks like it can be parsed a line at a time and doesn't really need to do any recursive parsing, so you could probably parse a stream of it as it arrives, which I imagine is trickier with JSON.