@yogieanamcara,
Thanks for writing. Yes, the various settings can be confusing. But with any complex plugin comes complex settings. I’ll try and answer your questions. But please also review the help which comes with the plugin.
Is it really simple to just exclude certain posts from the RSS Feeds and from appearing at the main post loop?
Yes, really simple. In wp-admin go to Simply-Exclude > Settings. The default view will show the taxonomies setup within your system. For each taxonomy (categories, post_tags, etc) you will find the actions listing showing the ability to include/exclude based on the action like Front/Home, RSS, Search, Archive. Just set the action you want. You can also go to the Posts > Categories page and manage specific categories. Or on Posts > All Posts to manage individual posts.
They should still of course appear on a search or category page!?
Yes, of course. When you exclude a category for example from the RSS feed action this does not automatically exclude it from the search of archive page. Those are separate actions.
As there are so many settings, are the default settings already hiding any posts, categories or anything else?
There are not really any ‘default’ setting. Meaning by default no specific categories or such are excluded from some action like home page or RSS. Those are for you to set.
Can I exclude posts on defining if they have a certain meta assigned?
No. The plugin does not query post meta values. This is just a way to exclude posts based on category, tag, etc.
Good luck.