The <!—more—> tag… What an annoyance.
Apparently, in a select few old versions of WordPress, the tag when used to split a post as to not flood the main page also affected the RSS feeds. In current versions they reverted this change due to complaints, even though there were a number of plugins available to display entire posts in RSS if you wanted that functionality. (I don’t.)
However, even with the complaints of the people who liked the ability to split their feed at a specific point, they haven’t bothered to restore it as an option. I haven’t found a plugin that’ll do it, either. ©Feed said in it’s description that it should but it doesn’t appear to work right. Nothing else even mentions anything except fixing the ‘bug’ with 2.1.