Jim,
Thanks for the explanation.
Basicall (in summary) ExtendedSyntaxParser will strip out HTML comments ... ... if an extension adds HTML comments to an article's rendered body, they'll be stripped and this can interfere with functionality. ... As far as I can tell, there's no good reason why ESP and TabbedData couldn't live in perfect harmony. I have no evidence to support Simon's assertion that they don't.
I can't see why they wouldn't; TabbedData doesn't use HTML comments. I just installed ExtendedSyntaxParser and UsenetSyntax into my wiki (which already has TabbedData), and I could use both UsenetSyntax and TabbedData in the same page, no problems. This is on MediaWiki 1.9.3.
Having said that, I got this error on all page loads (I have PHP error reporting enabled):
<b>Warning</b>: Call-time pass-by-reference has been deprecated; If you would like to pass it by reference, modify the declaration of xml_set_object(). If you would like to enable call-time pass-by-reference, you can set allow_call_time_pass_reference to true in your INI file. in <b>C:\WebServer\wiki-common\extensions\ExtendedSyntaxParser.php</b> on line <b>130</b><br />
Ian