Looking through the functions here, I found strip_html and truncatewords.
Here's a sample "posts list" with 75 words of preview text.
<ul >
{% for post in site.posts limit 4 %}
<li><span>{{ post.date | date_to_string }}</span> » <a href="{{ BASE_PATH }}{{ post.url }}">{{ post.title }}</a></li>
{{ post.content | strip_html | truncatewords:75}}<br>
<a href="{{ post.url }}">Read more...</a><br><br>
{% endfor %}
</ul>
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