Should you give all the posts in a Forum an unique description? Or let it empty so Google can make one with the crawled keywords .... ...

[Zanox]

To make all descriptions for all forum posts unique is a hell of a job....

One option is to crawl the first 165 characters and turn these automaticly into the meta description of the page.
If Google thinks the meta description is not suitable for the search query, Google will make a own description.

In this case all te meta descriptions are unique, like the Google Guidlines want you to do.

How will Google think off the fact when we delete the meta description tag so Google will make all the descriptions by herself?

[Zanox]

Hi Matt,

Thanks for your complete view on this topic.

You're right, it is not a crawling thing but dus adding a PHP line into the template

Cheers,

Wesley Kelder

[Zanox]

Thanks for your confirmation!

[matbennett]

If you are going to create a description then as Todd says, use the first 150-160 characters (note characters - not words).  However, you shouldn't need to crawl the site to do that, they should be generated automatically as part of your page template.

I presume your forum generates pages from content in a database. If that is the case (it almost always is) then you just need to adjust the template and the descriptions will automatically appear when the page is output.  A simple job for whoever looks after development for you. If you use a common forum platform there is almost certainly as "seo" plugin that handles this for you.

For forum category pages you can probably write these manually. This is where they are probably more likely to do good as well.  Personally I would focus on those and not worry too much about adding a meta-description to the thread pages.  If you don't mind tools like GWMT and MOZ flagging up the fact that they are missing, their absence is going to have very little impact compared with taking a snippet from the body text.

I know that some people will scream in shock at the thought of not having a meta-desc.  I do though notice that even Moz.com don't bother on thread pages (like this one)

[toddmumford]

I would recommend doing just what you suggested - utilize the first 152.