next()查找所选元素之后的下一个元素。它不再是content元素。下一个元素是您刚刚插入的段落。

$(this).parent().siblings('.content').slideToggle(250);

这是一个working jsfiddle。
除了第一个内容div之外，所有内容div最初都是隐藏的。不需要将其删除并放置在其他地方- CSS可以通过查找第一个section元素然后在其中查找.content div来找到它。
对于jQuery，click事件查找section父元素，然后在其中搜索.content div以切换显示。

nextonly 查看单个后续兄弟元素，然后应用过滤器。
现在简单的修复。将nextAll与.content过滤器配合使用：

$(this).find('h2 a').click(function() {
    $(this).parent().nextAll('.content').slideToggle(250);
    return false;
});

nextAll只查看后面的元素，所以当你知道后面的元素时，nextAll比siblings更有效。

我认为你的问题是内容的选择检查片段
你做了$（this）.parent（），它是h2标签而不是.content，然后使用find better来查找元素.content

$(this).parent().parent().find('.content').slideToggle(250);

// feed-panels-expanding
$('.feed-panels-expanding section').each(function() {

    $(this).find('h2 a').click(function() {
      //you did $(this).parent() witch is the h2 tag not the scetion and use find better
        $(this).parent().parent().find('.content').slideToggle(250);
        return false;
    });

    $(this).find('.content p:first').insertAfter($(this).find('h2'));
    $(this).find('.content').hide();

});

<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
<div class="feed-panels-expanding">
    <div class="feed">

        <section>

            <h2><a href="#">Heading One</a></h2>
            <p>Ut in nulla enim. Phasellus molestie magna non est bibendum non venenatis nisl tempor. Suspendisse dictum feugiat nisl ut dapibus. Mauris iaculis porttitor posuere. Praesent id metus massa, ut blandit odio. Proin.</p>

            <div class="content">
                <p>Duis aliquet egestas purus in blandit. Curabitur vulputate, ligula lacinia scelerisque tempor.</p>
                <p>Lacus lacus ornare ante, ac egestas est urna sit amet arcu. Class aptent taciti sociosqu ad litora torquent per conubia nostra, per inceptos himenaeos. Sed molestie augue sit amet leo consequat posuere. Vestibulum ante ipsum primis in faucibus orci luctus et ultrices posuere cubilia Curae;</p>
                <p>Proin vel ante a orci tempus eleifend ut et magna. Lorem ipsum dolor sit amet, consectetur adipiscing elit. Vivamus luctus urna sed urna ultricies ac tempor dui sagittis.</p>
            </div>

        </section>

        <section>

            <h2><a href="#">Heading Two</a></h2>
            <p>Ut in nulla enim. Phasellus molestie magna non est bibendum non venenatis nisl tempor. Suspendisse dictum feugiat nisl ut dapibus. Mauris iaculis porttitor posuere. Praesent id metus massa, ut blandit odio. Proin.</p>

            <div class="content">
                <p>Duis aliquet egestas purus in blandit. Curabitur vulputate, ligula lacinia scelerisque tempor.</p>
                <p>Lacus lacus ornare ante, ac egestas est urna sit amet arcu. Class aptent taciti sociosqu ad litora torquent per conubia nostra, per inceptos himenaeos. Sed molestie augue sit amet leo consequat posuere. Vestibulum ante ipsum primis in faucibus orci luctus et ultrices posuere cubilia Curae;</p>
                <p>Proin vel ante a orci tempus eleifend ut et magna. Lorem ipsum dolor sit amet, consectetur adipiscing elit. Vivamus luctus urna sed urna ultricies ac tempor dui sagittis.</p>
            </div>

        </section>

        <section>

            <h2><a href="#">Heading Three</a></h2>
            <p>Ut in nulla enim. Phasellus molestie magna non est bibendum non venenatis nisl tempor. Suspendisse dictum feugiat nisl ut dapibus. Mauris iaculis porttitor posuere. Praesent id metus massa, ut blandit odio. Proin.</p>

            <div class="content">
                <p>Duis aliquet egestas purus in blandit. Curabitur vulputate, ligula lacinia scelerisque tempor.</p>
                <p>Lacus lacus ornare ante, ac egestas est urna sit amet arcu. Class aptent taciti sociosqu ad litora torquent per conubia nostra, per inceptos himenaeos. Sed molestie augue sit amet leo consequat posuere. Vestibulum ante ipsum primis in faucibus orci luctus et ultrices posuere cubilia Curae;</p>
                <p>Proin vel ante a orci tempus eleifend ut et magna. Lorem ipsum dolor sit amet, consectetur adipiscing elit. Vivamus luctus urna sed urna ultricies ac tempor dui sagittis.</p>
            </div>

        </section>

    </div>
</div>

我想你想要的是 accordion 。
更多信息：http://inspirationalpixels.com/tutorials/creating-an-accordion-with-html-css-jquery