A few miscellaneous thoughts:
I would consider dropping the "& Widget" part of the name, because this doesn't appear to be a widget.
Being able to put a shortcode inside a text widget, as illustrated in the below screenshot that I found on your plugin page, doesn't qualify as a widget.
Side note, I'm guessing code is added to the plugin that enables shortcodes inside text widgets? Because they are not parsed by default.
If so, I would make sure the code you add just parses your [recentposts-sc] shortcode, and not all of them like, if it doesn't already do that.
The plugin Recent Posts Widget Extended is a good example of a standalone recent posts widget.
You're instructing users to manually fill out all the shortcode attributes. Your "combination of several parameters" example highlights how overwhelming this can be:
[recentposts-sc numberofposts=”7″ post_type=”post” get_cat_name=”true” category=”lesson” author=”admin” excerpt_more=”true” label=”Read More…” after=”January 5th, 2015″ before=”October 6th, 2015″ include_author=”true” orderby=”name” order=”DSC” excerptlength=30 post_ids=”181,189,194″ offset=”0″ tag=”lesson1,lesson2,lesson” meta_key=”size” meta_value=”small, medium”]
This could be user-friendlier with a shortcode builder UI within the post editor.
I can't find a good example of a tutorial, but something like this should get you started. That'll at least get the button in TinyMCE, but I'm thinking a UI similar to Recent Posts Widget Extended, that just builds out the shortcode with attributes.
The plugin banner, along with the default styles, don't look professional. I'd consider hiring a designer to first make new default styles for the plugin, and then updating the plugin banner to include the updated default styles.
There should be a filter to turn off default CSS, if there isn't already, so developers can programmatically turn it off without needing to disable it in the admin.
Speaking of filters, it looks like in one of the reviews, someone complained about not being able to adjust the markup with filters. This should be another developer feature.
You can make the markup super flexible, but someone will come along and find that it's not flexible enough.
Promotion is a tricky subject. You could try reaching out to WordPress news sites to see if they'd cover it, but I would probably wait until it's a bit more user-friendly before doing that. Lots of past discussions about that here. Try starting with this one: Tips for promoting a free WordPress plugin
I don't recall Recent Posts Widget Extended having any promotion, however. I think it's possible to have a plugin naturally grow with no other promotion outside of WordPress.org, but that's easier said than done.
Best of luck!