The problem was that an e-mail enabled InfoPath Forms Library takes in the To, CC, and Subject fields but does not take in the Body field. And since it's not an XML file but rather an .eml file, it's a little ugly for stripping out body text. But hey, a Discussion List includes that body text, great! Except the problem with that list seems to be that emails to it do not autostart Workflows on new item created. There is a reason for this lack of behavior on a Discussion List but doesn't really explain why that same logic doesn't apply to the other lists. Anyway, the short of it is run this command:

stsadm.exe –o setproperty –propertyname declarativeworkflowautostartonemailenabled –propertyvalue yes

You will also need to have installed this update PRIOR TO running that command. Some have mentioned setting the System Account with full permissions but that didn't seem to affect anything.

Someone needs to create a “Superlist” in SharePoint that includes all of the features of all of the other lists. And one of these days I'm going to create a table to show what each list can/can't do.