Plugins should not hijack the blog admin. It is fine to include an Upgrade prompt on the plugin admin page, but not throughout the blog. It is acceptable to embed a widget on the dashboard but this should be the same size as others and be dismissable. It s fine to put an error message at the top of the admin for special cases, but it should be linked to a way to fix the error and it should be infrequent. Any form of nagging is absolutely prohibited.