Exports aren't working if you are using a PHP version less than 5.5. The problem is line 435 in CRM/Export/BAO/Export.php:
This will give an error in all PHP versions below 5.5. From the PHP manual (http://php.net/manual/en/function.empty.php): "Prior to PHP 5.5, empty() only supports variables; anything else will result in a parse error"
You should be able to fix this by replacing lines 435-437 in Export.php with something like:
This issue has also been reported on StackExchange: http://civicrm.stackexchange.com/questions/10569/help-fatal-error-doing-an-export-civi-4-7-4