I'm still a little unclear as to what was added in entrenchment as I didn't play vanilla sins. I do know the entire fortification/defense tree was added, but besides that I have no clue.
The defense research tree was the big thing that was added. They also improved the AI, now it will go after nuetral extractors, avoid mines and tactical structures better, and AI surrender is much better. If you're really interested, you should be able to search the old change logs from the forums.