List Category Posts rewrite necessitates rewrite of customised templates

Thank you for reporting on this issue. Regarding the template system, I once more updated it, thinking the new version is easier to the user. I know it still lacks a big rewrite to make it user-friendly. The underlying code is pretty much set to use any new idea for it.

I added some upgrade notices for version 0.17 and 0.18. I guess it wasn’t clear enough. I don’t know if this is the place to discuss this, since it’s more of a question / answer site (sorry WP Answers if this is wrong), but what would you think of a system like this?

[catlist id=6]
  <h2>{post-title}</h2>
  <i>Date: {date}</i>
  <p class="excerpt">{excerpt}</p>
[/catlist]

That would be simpler to write and understand for users who don’t know PHP.

Regarding the thumbnails, I’ve tested it several times and it is working for me. You should add a new question on this site, with all the info on your theme, WordPress installation and the functions.php code for thumbnails, see if we can get other user / developer figure out why the plugin is not displaying the thumbnail.