The markup must provide ways to search and mark text as data, numeric, token, text. All of them should have ways to encode fine details in them.
The markup should be usable at first try, added feature should not break that. If someone new to it is having hard time to understand why the way he wrotes the text fail or having lot of error while writing simple text, there is high probability that the markup should change.
It should have possibilities to encode complex writing and relations between part of texts, having something permiting meta writing could do the trick.
It should keep the basic writing features very easy to read, to compare it should be a bit more readable than Markdown file format.
The mark up should not fail when we write raw text, warnings will be logged but the parsing will not.
Having very detailed logs is not the goal yet, but it must catch error with precise location and avoid stack trace like Java.
No compromise should be made on the performance, the parser should be able to parse hundreds of file, checks their links in at least less than ten seconds, but much less is the goal.
IDK must produce HTML which is compliant to the specs, the output is checked regularly to see if it does.