TOML would be great, if not for an annoying obscure detail in the specification that makes it hard to use for my typical use cases (scientific computation) [1]. Moreover, I find quite unintuitive how you are supposed to specify array of tables [2]: this kind of is much easier in JSON (which is the format I am currently using, although it is far from perfect).
That is an annoyingly obscure detail, I think the wrong decision was made there. Hopefully it gets reversed.
Personally I really like the array of tables syntax. It is a little unintuitive but it's not difficult to remember. It's useful for fulfilling the OP's "No document closing necessary, so appending is trivial" requirement.
And if you don't want to use it, you can always use inline tables in an array, just like JSON.
[1] https://github.com/toml-lang/toml/issues/356
[2] https://github.com/toml-lang/toml#user-content-array-of-tabl...