I have a site that has posts without titles. You can think of them as being a lot like Twitter tweets.
Should I put the post contents in the <title> tag or the meta description tag? If I put the post contents in one of the tags what should I put in the other?
My challenge is that we have very short amounts of content with no titles. I want to avoid having too many duplicate titles or descriptions.
We have things like user name, full name, date, etc.
If I were in your shoes, I would write custom title and description for every little post. Not large, but in a few words. It will help you to solve the problem with the duplicate meta tags ans shows Google, that your site if user-friendly.
A title tag should be present on every page so that when a visitor views your SERP, or bookmarks a page, they'll see a concise description that reflects the contents of it.
As you stated, you should avoid duplicate titles, that is titles which are repetitive or boilerplate. Search engines do allow for branding however, for example:
<title>Stack Exchange (StackExchange) on Twitter</title>
In the above, "Twitter" is the brand, and the unique portion is the username "StackExchange". So using this as an example for the pages containing posts on your site, you can have your brand along with a unique username, or similar as related to the poster or subject.
For more information on Google's suggested title usage, see the section named "Create descriptive page titles" here: Google Webmaster Tools: Site title and description
Also note that you can view problems with your site's titles by following the steps here: Google Webmaster Tools: HTML Improvements
For reasons why to avoid duplicate meta tag descriptions, see the section named "Avoid Duplicate Meta Description Tags" here: Meta Description
Actually if you have a lot of pages indexed in the search engines then you are most likely okay not having titles on those certain pages for the time being. It may hurt your search results down the road, you can also exclude them in robots.txt. If there is not <title> tag Google will pick the most relevant title it believes the page should have based on the design and the content.
