External links in Parser output have <a rel="nofollow" class="external";
external links in Parsoid output have <a title="" rel="mw:ExtLink"
Outputting rel="nofollow" tells search engines not to follow the link (more a suggestion to Google) and is an indicator to decrease search engine rank. So adding this to Flow's HTML output from Parsoid might reduce spammers' incentive.
There's logic in Parser::getExternalLinkRel to decide whether to output nofollow, I think Flow can just call that public function in code similar to includes/Parsoid/Redlinker.php.
<gwicke> do you remember what the use case for the exceptions was?
$wgNoFollowNsExceptions and $wgNoFollowDomainExceptions
<hoo> First one: No idea... probably someone thought someone else could use that sometime.
Second one: To white list own domains