This episode used to be one of my all-time favorites. I still love this episode a lot, but it does lack the endearing quality of those of the earlier seasons (which someone pointed out), so I give it a 9/10.
There are just so many great Simpsons moments in this episode, SO MANY, which is why i love it so much. The best for me was probably "Take that, Lisa's beliefs!" after Bart zings Lisa by stealthy placing a Thai door-hanger menu on her nose. Also, the "Lisa's Dead! ..Unless.." is hilarious as well as the Kentucky Fried Panda and "Daughter now goes to state college, baseball caps everywhere!", and of course the closing song with the Lisa Log going cross-country!!
Ahh so many classic Simpsons moments! All of these combined with a very entertaining, spontaneous (if not exactly coherent, but that's part of the charm!) plot makes "Lisa the Tree Hugger" one of my favorite episodes.