For usability purposes, entire article thumbnail is wrapped to a link.

<a href="/some_article">
   <h2>Article title</h2>
   <div class="summary">Lorem ipsum dolor sit amet</div>
</a>

User needs to click on any place of a thumb and it will be redirected to article.

Does this approach have some negative effect to SEO ?

Another question:

What is more valueable for Search Engine ?

Just a link to article in articles list

<a href="/article1">Article 1</a>
<a href="/article2">Article 2</a>
<a href="/article3">Article 3</a>    

Or h2, wrapped to link:

<a href="/article1"><h2>Article 1</h2></a>
<a href="/article2"><h2>Article 2</h2></a>
<a href="/article3"><h2>Article 3</h2></a>