I don't use Second Life facial expressions much. They remind me of the scene on ST:TNG in which Data tries to smile. (Darn; can't find the clip.)
Perhaps that will change someday, though. You should head right over to New World Notes and read "Animated Mesh Heads May Be the Expressive Face of Tomorrow's SL Avatar", and above all, watch the videos! Shirousagi Noel of Snow Rabbit has done some amazing work.
To be sure, there's the problem of customization--will people choose a face that is theirs, the result of slider tweaking to one's satisfaction, or one that is beautifully expressive but, modulo skin tones and hair and eye color, looks just like all the other beautifully expressive faces? Fix that problem, and Second Life will be that much more immersive.