To sum up FS#260
and this one (FS#208
What is not allowed in headers
2. formatting (italic, bold, underline, code, ...)
3. typography ((c), =>, ---, ...) and smileys (which have not been mentioned before)
4. other things which not have been requested (yet), but which fall into this same category (footnotes, images, lists, tables, ...)
IMHO the first, second and fourth issue being disallowed, makes absolutely sense; whereas the third one does not.
1. There is no reason I could think of that requires links in headlines. Allowing this could tempt people to misuse headlines.
2. Headlines of the same rank should always look the same, allowing them to be recognized as such. If you want to change the overall look of a certain headline type, change the CSS.
3. Allowing typography (and smileys perhaps) in headlines makes perfectly sense to me. Is there a reason why e.g. '->' should not be displayed as '→'? The only thing which could probably interfere with the equal signs of the headline syntax is '=>' or '<='!?
4. I hope there is no need to explain why images, lists, quoting and tables should not be allowed in headlines ...