The Alien Resurrection extended cut is nothing special. Besides the pointless new opening, all I can remember is some expanded lines (thank goodness for the little logo in the corner during new shots or you wouldn't notice).

For example:

Ripley is asking if the scientists work for Weyland-Yutani. In the theatrical cut, they mention that it went under a long time ago. In the extended they add the line "They were bought out by Walmart."

I've not seen the movie since I started liking Joss Whedon, so its possible that all of these expanded lines are just Joss showing through the awfulness they covered his script in.