I just upgraded my Geeklog from 1.3.9 to 1.4.0sr1. I had RSS syndication set up with "Length of Entries" set to 1, to include the full intro text of the article. Now, instead of the full intro text, it includes the entire article. Is there some way to make it go back to just the intro text? I tried 0, and that gives no text at all. I don't really want to cut it off at some arbitrary number of characters.

It has been a year, you probably have solved this, but someone may still need.
I had the same problem so I checked the lib-syndication.php
Find this line (line 371 in 1.4.1):

remove the bodytext part:

seems to work for me.
Make sure you backup your original file so that you can go back. Also, one could probably make a $_CONF variable and add the option of having whole body or just the intro.

Works for me too. I actually changed it in two places in lib-syndication.php: line 263 and line 371. I didn't dig deeply enough to figure out what the two different sections of code are doing, but I figured it would be safest to change both of them.