Newlines in posts are not converted to html linebreak tags, and therefore not visible as new lines in e-mails sent out through MailChimp. This makes the e-mail really hard to read, with no paragraphs, just a huge block of text.
This is easily fixed by converting newlines to html using for example the nl2br() function.
This is how I did it (this is on version 1.25 of the plugin, I can't get version 2.0 to work):
functions.php, line 262:
$content = array(
'html' => nl2br($content),
'generate_text' => true,