MarketPress causes comments to be artificially turned off. Might be because of this PHP error:
"NOTICE: wp-content/plugins/marketpress/includes/public/class-mp-public.php:86 - Undefined index: pages
require('wp-blog-header.php'), require_once('wp-includes/template-loader.php'), include('/themes/coursepress-child/front-page.php'), get_header, locate_template, load_template, require_once('/plugins/coursepress/themes/coursepress/header.php'), body_class, get_body_class, apply_filters('body_class'), call_user_func_array, MP_Public->add_mp_body_class"
I'm not trying to be wise but do these plugins get tested before shipping? This is the second instance of a WPMU plugin completely debilitating core WordPress functionality that I've come across in the past month.