after creating a child theme for Twenty Eleven, copy content.php from the Twenty Eleven theme into your child theme to edit it;
find (line 35):
<?php if ( is_search() ) : // Only display Excerpts for Search ?>
change to:
<?php if ( is_search() || is_paged() || $wp_query->current_post > 0 ) : // Only display Excerpts for Search, paginated pages, or any other then the first post ?>
then find this section:
<div class="entry-content">
<?php the_content( __( 'Continue reading <span class="meta-nav">→</span>', 'twentyeleven' ) ); ?>
and change to:
<div class="entry-content">
<?php global $more; if( $wp_query->current_post == 0 && !is_paged() ) $more = 1; ?>
<?php the_content( __( 'Continue reading <span class="meta-nav">→</span>', 'twentyeleven' ) ); ?>