The length of page titles is restricted to 70 characters
Ideally page titles should be shorter than the 70 characters allowed - the current recommendation from W3C is 64 characters.
There are a couple of reasons for this restriction. The first is related to SEO as Google will mark down pages which have page titles which it considers to be excessively long.
Another reason, related to SEO, is that search engines will truncate long page titles to fit them into the search engine results pages. There’s a good summary of what the different search engines recommend at http://blog.powermapper.com/blog/post/Page-Title-Length-for-Search-Engines.aspx.
If search engines don’t display the page title in full then it’s going to be more difficult to get people to click through to the page.
Also, for accessibility reasons it’s best to have short page titles. On pages where much of the content is made up of the titles of other pages (e.g. a search results page or blog landing page) it would take someone using a screen reader longer to get an idea of what your pages are about if the page titles are long.