I am still looking for my “ideal” WordPress.com theme and have been using the “Feature Features” link (via the “themes.php” webpage of my Dashboard) to narrow down my selections to include, for example, “Post Formats” and “Infinite Scroll”.
But I’d like to see a label for feature id “Excerpts” as part of the theme filter.
In the meantime, I’ve been using both the “More” tag and the “post excerpt box” of my “edit.php” page.
Infinite scroll is a site wide change and Staff will be implementing it progressively on all WordPress.com themes that have a suitable structure for infinite scroll. See here for infinite scroll themes updates.