Why do you let people choose the title? Wouldn't it be easier to grab it from the page's title tag automatically?

I'd say while this might help, titles need editing by the submitter more often than not, be that for removing some extra tags/snippets/etc from the title, simplifying/abbreviating long titles, etc. (which is usually ok if you don't editorialize)

That alas is the sort of thing that works 80% of the time and fails the other 80% of the time.

What about a feature where the title is automatically populated and the user can modify it if incorrect?

The HN bookmarklet (in the footer) does that:

https://news.ycombinator.com/bookmarklet.html

Then half the submissions here would be “Github.com”.