Question: For example, I want to play all 58 videos uploaded by a single user.

Obviously I could create a playlist, add each video to it, then play the entire playlist, but is there a simpler way?

Answer: 
Little trick I found is to get a user's unique code/string from their profile page's source. Here's how to do it:

* Go to their user page and right-click -> "view page source".
* Search for "vnd" and copy the code that you find (It should start with "UC").
* Open one of the user's videos. In the URL address, add the following to the end: "&list=" then paste the string you copied.
* Final step is simply change the 2nd character of the code you pasted from a C to a U. Hit enter and the video should reload, and now it appears in a playlist of all the user's videos.

This seems to work pretty reliably, but sometimes does not get all of the user's videos.
The Firefox add-on called [YouTube Play All](https://addons.mozilla.org/en-US/firefox/addon/222206/) seems to do what you want.